cpython-lru_cache-weakref: Doc/tutorial/introduction.rst annotate

annotate Doc/tutorial/introduction.rst @ 83447:678564fc0962 3.3

#17789: test_random now works with unittest test discovery. Patch by Zachary Ware.

author	Ezio Melotti <ezio.melotti@gmail.com>
date	Fri, 19 Apr 2013 05:45:27 +0300
parents	66a22b6a3414
children	796d1371605d

rev	line source
66161 ec42112c2100 Backport rev86422 for issue 10304 from py3k to 31-maint Terry Reedy <tjreedy@udel.edu> parents: 66076 diff changeset	1 .. _tut-informal:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	2
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	3 **********************************
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	4 An Informal Introduction to Python
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	5 **********************************
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	6
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	7 In the following examples, input and output are distinguished by the presence or
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	8 absence of prompts (``>>>`` and ``...``): to repeat the example, you must type
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	9 everything after the prompt, when the prompt appears; lines that do not begin
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	10 with a prompt are output from the interpreter. Note that a secondary prompt on a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	11 line by itself in an example means you must type a blank line; this is used to
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	12 end a multi-line command.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	13
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	14 Many of the examples in this manual, even those entered at the interactive
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	15 prompt, include comments. Comments in Python start with the hash character,
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	16 ``#``, and extend to the end of the physical line. A comment may appear at the
c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	17 start of a line or following whitespace or code, but not within a string
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43815 diff changeset	18 literal. A hash character within a string literal is just a hash character.
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	19 Since comments are to clarify code and are not interpreted by Python, they may
c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	20 be omitted when typing in examples.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	21
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	22 Some examples::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	23
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	24 # this is the first comment
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	25 SPAM = 1 # and this is the second comment
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	26 # ... and now a third!
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	27 STRING = "# This is not a comment."
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	28
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	29
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	30 .. _tut-calculator:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	31
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	32 Using Python as a Calculator
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	33 ============================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	34
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	35 Let's try some simple Python commands. Start the interpreter and wait for the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	36 primary prompt, ``>>>``. (It shouldn't take long.)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	37
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	38
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	39 .. _tut-numbers:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	40
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	41 Numbers
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	42 -------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	43
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	44 The interpreter acts as a simple calculator: you can type an expression at it
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	45 and it will write the value. Expression syntax is straightforward: the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	46 operators ``+``, ``-``, ``*`` and ``/`` work just like in most other languages
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	47 (for example, Pascal or C); parentheses can be used for grouping. For example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	48
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	49 >>> 2+2
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	50 4
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	51 >>> # This is a comment
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	52 ... 2+2
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	53 4
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	54 >>> 2+2 # and a comment on the same line as code
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	55 4
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	56 >>> (50-5*6)/4
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	57 5.0
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	58 >>> 8/5 # Fractions aren't lost when dividing integers
54927 e3025a4c0ad7 Merged revisions 73636 via svnmerge from Mark Dickinson <dickinsm@gmail.com> parents: 54548 diff changeset	59 1.6
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	60
50563 93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	61 Note: You might not see exactly the same result; floating point results can
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	62 differ from one machine to another. We will say more later about controlling
54931 d94b3ed5d536 Merged revisions 73640 via svnmerge from Mark Dickinson <dickinsm@gmail.com> parents: 54927 diff changeset	63 the appearance of floating point output. See also :ref:`tut-fp-issues` for a
d94b3ed5d536 Merged revisions 73640 via svnmerge from Mark Dickinson <dickinsm@gmail.com> parents: 54927 diff changeset	64 full discussion of some of the subtleties of floating point numbers and their
d94b3ed5d536 Merged revisions 73640 via svnmerge from Mark Dickinson <dickinsm@gmail.com> parents: 54927 diff changeset	65 representations.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	66
50563 93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	67 To do integer division and get an integer result,
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	68 discarding any fractional result, there is another operator, ``//``::
50563 93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	69
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	70 >>> # Integer division returns the floor:
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	71 ... 7//3
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	72 2
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	73 >>> 7//-3
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	74 -3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	75
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	76 The equal sign (``'='``) is used to assign a value to a variable. Afterwards, no
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	77 result is displayed before the next interactive prompt::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	78
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	79 >>> width = 20
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	80 >>> height = 5*9
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	81 >>> width * height
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	82 900
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	83
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	84 A value can be assigned to several variables simultaneously::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	85
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	86 >>> x = y = z = 0 # Zero x, y and z
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	87 >>> x
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	88 0
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	89 >>> y
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	90 0
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	91 >>> z
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	92 0
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	93
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	94 Variables must be "defined" (assigned a value) before they can be used, or an
c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	95 error will occur::
c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	96
79153 5f4841977bee Issue #16015: Fix NameError doctest example in tutorial introduction. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 78507 diff changeset	97 >>> n # try to access an undefined variable
50563 93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	98 Traceback (most recent call last):
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	99 File "<stdin>", line 1, in <module>
c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	100 NameError: name 'n' is not defined
c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	101
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	102 There is full support for floating point; operators with mixed type operands
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	103 convert the integer operand to floating point::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	104
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	105 >>> 3 * 3.75 / 1.5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	106 7.5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	107 >>> 7.0 / 2
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	108 3.5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	109
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	110 Complex numbers are also supported; imaginary numbers are written with a suffix
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	111 of ``j`` or ``J``. Complex numbers with a nonzero real component are written as
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	112 ``(real+imagj)``, or can be created with the ``complex(real, imag)`` function.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	113 ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	114
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	115 >>> 1j * 1J
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	116 (-1+0j)
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43157 diff changeset	117 >>> 1j * complex(0, 1)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	118 (-1+0j)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	119 >>> 3+1j*3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	120 (3+3j)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	121 >>> (3+1j)*3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	122 (9+3j)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	123 >>> (1+2j)/(1+1j)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	124 (1.5+0.5j)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	125
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	126 Complex numbers are always represented as two floating point numbers, the real
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	127 and imaginary part. To extract these parts from a complex number z, use
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	128 ``z.real`` and ``z.imag``. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	129
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	130 >>> a=1.5+0.5j
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131 >>> a.real
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	132 1.5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	133 >>> a.imag
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	134 0.5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	135
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	136 The conversion functions to floating point and integer (:func:`float`,
43449 7fcd9945d1bf #1211, #1212, #1213: py3k fixes to the tutorial. Georg Brandl <georg@python.org> parents: 43201 diff changeset	137 :func:`int`) don't work for complex numbers --- there is not one correct way to
7fcd9945d1bf #1211, #1212, #1213: py3k fixes to the tutorial. Georg Brandl <georg@python.org> parents: 43201 diff changeset	138 convert a complex number to a real number. Use ``abs(z)`` to get its magnitude
7fcd9945d1bf #1211, #1212, #1213: py3k fixes to the tutorial. Georg Brandl <georg@python.org> parents: 43201 diff changeset	139 (as a float) or ``z.real`` to get its real part::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	140
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	141 >>> a=3.0+4.0j
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	142 >>> float(a)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	143 Traceback (most recent call last):
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	144 File "<stdin>", line 1, in ?
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	145 TypeError: can't convert complex to float; use abs(z)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	146 >>> a.real
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	147 3.0
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	148 >>> a.imag
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	149 4.0
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	150 >>> abs(a) # sqrt(a.real2 + a.imag2)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	151 5.0
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	152
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	153 In interactive mode, the last printed expression is assigned to the variable
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	154 ``_``. This means that when you are using Python as a desk calculator, it is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	155 somewhat easier to continue calculations, for example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	156
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	157 >>> tax = 12.5 / 100
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	158 >>> price = 100.50
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	159 >>> price * tax
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	160 12.5625
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	161 >>> price + _
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	162 113.0625
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	163 >>> round(_, 2)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	164 113.06
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	165
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	166 This variable should be treated as read-only by the user. Don't explicitly
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	167 assign a value to it --- you would create an independent local variable with the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	168 same name masking the built-in variable with its magic behavior.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	169
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	170
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	171 .. _tut-strings:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	172
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	173 Strings
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	174 -------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	175
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	176 Besides numbers, Python can also manipulate strings, which can be expressed in
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	177 several ways. They can be enclosed in single quotes or double quotes::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	178
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	179 >>> 'spam eggs'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	180 'spam eggs'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	181 >>> 'doesn\'t'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	182 "doesn't"
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	183 >>> "doesn't"
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	184 "doesn't"
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	185 >>> '"Yes," he said.'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	186 '"Yes," he said.'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	187 >>> "\"Yes,\" he said."
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	188 '"Yes," he said.'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	189 >>> '"Isn\'t," she said.'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	190 '"Isn\'t," she said.'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	191
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	192 The interpreter prints the result of string operations in the same way as they
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	193 are typed for input: inside quotes, and with quotes and other funny characters
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	194 escaped by backslashes, to show the precise value. The string is enclosed in
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	195 double quotes if the string contains a single quote and no double quotes, else
66076 96c80f42bce6 Merged revisions 86310 via svnmerge from Senthil Kumaran <orsenthil@gmail.com> parents: 66075 diff changeset	196 it's enclosed in single quotes. The :func:`print` function produces a more
66075 6d1294adfaf4 Merged revisions 86310 via svnmerge from Senthil Kumaran <orsenthil@gmail.com> parents: 62962 diff changeset	197 readable output for such input strings.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	198
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	199 String literals can span multiple lines in several ways. Continuation lines can
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	200 be used, with a backslash as the last character on the line indicating that the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	201 next line is a logical continuation of the line::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	202
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	203 hello = "This is a rather long string containing\n\
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	204 several lines of text just as you would do in C.\n\
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	205 Note that whitespace at the beginning of the line is\
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	206 significant."
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	207
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	208 print(hello)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	209
62962 365eeab4a350 Merged revisions 82301 via svnmerge from Georg Brandl <georg@python.org> parents: 56218 diff changeset	210 Note that newlines still need to be embedded in the string using ``\n`` -- the
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	211 newline following the trailing backslash is discarded. This example would print
55913 3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	212 the following:
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	213
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	214 .. code-block:: text
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	215
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	216 This is a rather long string containing
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	217 several lines of text just as you would do in C.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	218 Note that whitespace at the beginning of the line is significant.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	219
55913 3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	220 Or, strings can be surrounded in a pair of matching triple-quotes: ``"""`` or
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	221 ``'''``. End of lines do not need to be escaped when using triple-quotes, but
66161 ec42112c2100 Backport rev86422 for issue 10304 from py3k to 31-maint Terry Reedy <tjreedy@udel.edu> parents: 66076 diff changeset	222 they will be included in the string. So the following uses one escape to
ec42112c2100 Backport rev86422 for issue 10304 from py3k to 31-maint Terry Reedy <tjreedy@udel.edu> parents: 66076 diff changeset	223 avoid an unwanted initial blank line. ::
55913 3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	224
66161 ec42112c2100 Backport rev86422 for issue 10304 from py3k to 31-maint Terry Reedy <tjreedy@udel.edu> parents: 66076 diff changeset	225 print("""\
55913 3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	226 Usage: thingy [OPTIONS]
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	227 -h Display this usage message
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	228 -H hostname Hostname to connect to
56218 149bed21735e Merged revisions 75059 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 55913 diff changeset	229 """)
55913 3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	230
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	231 produces the following output:
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	232
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	233 .. code-block:: text
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	234
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	235 Usage: thingy [OPTIONS]
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	236 -h Display this usage message
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	237 -H hostname Hostname to connect to
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	238
52892 792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 50563 diff changeset	239 If we make the string literal a "raw" string, ``\n`` sequences are not converted
792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 50563 diff changeset	240 to newlines, but the backslash at the end of the line, and the newline character
792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 50563 diff changeset	241 in the source, are both included in the string as data. Thus, the example::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	242
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	243 hello = r"This is a rather long string containing\n\
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	244 several lines of text much as you would do in C."
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	245
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	246 print(hello)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	247
55913 3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	248 would print:
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	249
3fb1c1fbe26c Merged revisions 74745 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 54931 diff changeset	250 .. code-block:: text
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	251
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	252 This is a rather long string containing\n\
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	253 several lines of text much as you would do in C.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	254
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	255 Strings can be concatenated (glued together) with the ``+`` operator, and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	256 repeated with ``*``::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	257
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	258 >>> word = 'Help' + 'A'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	259 >>> word
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	260 'HelpA'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	261 >>> '<' + word*5 + '>'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	262 '<HelpAHelpAHelpAHelpAHelpA>'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	263
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	264 Two string literals next to each other are automatically concatenated; the first
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	265 line above could also have been written ``word = 'Help' 'A'``; this only works
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	266 with two literals, not with arbitrary string expressions::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	267
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	268 >>> 'str' 'ing' # <- This is ok
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	269 'string'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	270 >>> 'str'.strip() + 'ing' # <- This is ok
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	271 'string'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	272 >>> 'str'.strip() 'ing' # <- This is invalid
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	273 File "<stdin>", line 1, in ?
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	274 'str'.strip() 'ing'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	275 ^
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	276 SyntaxError: invalid syntax
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	277
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	278 Strings can be subscripted (indexed); like in C, the first character of a string
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	279 has subscript (index) 0. There is no separate character type; a character is
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43157 diff changeset	280 simply a string of size one. As in the Icon programming language, substrings
4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43157 diff changeset	281 can be specified with the slice notation: two indices separated by a colon.
4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43157 diff changeset	282 ::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	283
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	284 >>> word[4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	285 'A'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	286 >>> word[0:2]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	287 'He'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	288 >>> word[2:4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	289 'lp'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	290
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	291 Slice indices have useful defaults; an omitted first index defaults to zero, an
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	292 omitted second index defaults to the size of the string being sliced. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	293
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	294 >>> word[:2] # The first two characters
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	295 'He'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	296 >>> word[2:] # Everything except the first two characters
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	297 'lpA'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	298
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	299 Unlike a C string, Python strings cannot be changed. Assigning to an indexed
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	300 position in the string results in an error::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	301
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	302 >>> word[0] = 'x'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	303 Traceback (most recent call last):
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	304 File "<stdin>", line 1, in ?
54133 e8f1a007771a Merged revisions 72703 via svnmerge from Georg Brandl <georg@python.org> parents: 52892 diff changeset	305 TypeError: 'str' object does not support item assignment
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	306 >>> word[:1] = 'Splat'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	307 Traceback (most recent call last):
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	308 File "<stdin>", line 1, in ?
54133 e8f1a007771a Merged revisions 72703 via svnmerge from Georg Brandl <georg@python.org> parents: 52892 diff changeset	309 TypeError: 'str' object does not support slice assignment
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	310
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	311 However, creating a new string with the combined content is easy and efficient::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	312
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	313 >>> 'x' + word[1:]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	314 'xelpA'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	315 >>> 'Splat' + word[4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	316 'SplatA'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	317
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	318 Here's a useful invariant of slice operations: ``s[:i] + s[i:]`` equals ``s``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	319 ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	320
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	321 >>> word[:2] + word[2:]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	322 'HelpA'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	323 >>> word[:3] + word[3:]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	324 'HelpA'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	325
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	326 Degenerate slice indices are handled gracefully: an index that is too large is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	327 replaced by the string size, an upper bound smaller than the lower bound returns
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	328 an empty string. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	329
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	330 >>> word[1:100]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	331 'elpA'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	332 >>> word[10:]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	333 ''
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	334 >>> word[2:1]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	335 ''
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	336
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	337 Indices may be negative numbers, to start counting from the right. For example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	338
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	339 >>> word[-1] # The last character
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	340 'A'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	341 >>> word[-2] # The last-but-one character
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	342 'p'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	343 >>> word[-2:] # The last two characters
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	344 'pA'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	345 >>> word[:-2] # Everything except the last two characters
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	346 'Hel'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	347
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	348 But note that -0 is really the same as 0, so it does not count from the right!
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	349 ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	350
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	351 >>> word[-0] # (since -0 equals 0)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	352 'H'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	353
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	354 Out-of-range negative slice indices are truncated, but don't try this for
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	355 single-element (non-slice) indices::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	356
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	357 >>> word[-100:]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	358 'HelpA'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	359 >>> word[-10] # error
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	360 Traceback (most recent call last):
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	361 File "<stdin>", line 1, in ?
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	362 IndexError: string index out of range
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	363
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	364 One way to remember how slices work is to think of the indices as pointing
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	365 between characters, with the left edge of the first character numbered 0.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	366 Then the right edge of the last character of a string of n characters has
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	367 index n, for example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	368
50563 93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	369 +---+---+---+---+---+
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	370 \| H \| e \| l \| p \| A \|
50563 93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	371 +---+---+---+---+---+
93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	372 0 1 2 3 4 5
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	373 -5 -4 -3 -2 -1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	374
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	375 The first row of numbers gives the position of the indices 0...5 in the string;
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	376 the second row gives the corresponding negative indices. The slice from i to
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	377 j consists of all characters between the edges labeled i and j,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	378 respectively.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	379
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	380 For non-negative indices, the length of a slice is the difference of the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	381 indices, if both are within bounds. For example, the length of ``word[1:3]`` is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	382 2.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	383
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	384 The built-in function :func:`len` returns the length of a string::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	385
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	386 >>> s = 'supercalifragilisticexpialidocious'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	387 >>> len(s)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	388 34
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	389
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	390
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	391 .. seealso::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	392
79681 06deefec71e3 Fix and add more links. Ezio Melotti <ezio.melotti@gmail.com> parents: 79153 diff changeset	393 :ref:`textseq`
50563 93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	394 Strings are examples of sequence types, and support the common
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	395 operations supported by such types.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	396
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	397 :ref:`string-methods`
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	398 Strings support a large number of methods for
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	399 basic transformations and searching.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	400
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	401 :ref:`string-formatting`
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44570 diff changeset	402 Information about string formatting with :meth:`str.format` is described
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44570 diff changeset	403 here.
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44570 diff changeset	404
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44570 diff changeset	405 :ref:`old-string-formatting`
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44570 diff changeset	406 The old formatting operations invoked when strings and Unicode strings are
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44570 diff changeset	407 the left operand of the ``%`` operator are described in more detail here.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	408
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	409
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	410 .. _tut-unicodestrings:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	411
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	412 About Unicode
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	413 -------------
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	414
78507 3654c711019a Issue #15444: Use proper spelling for non-ASCII contributor names. Antoine Pitrou <solipsis@pitrou.net> parents: 74178 diff changeset	415 .. sectionauthor:: Marc-André Lemburg <mal@lemburg.com>
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	416
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	417
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	418 Starting with Python 3.0 all strings support Unicode (see
c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48655 diff changeset	419 http://www.unicode.org/).
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	420
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	421 Unicode has the advantage of providing one ordinal for every character in every
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	422 script used in modern and ancient texts. Previously, there were only 256
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	423 possible ordinals for script characters. Texts were typically bound to a code
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	424 page which mapped the ordinals to script characters. This lead to very much
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	425 confusion especially with respect to internationalization (usually written as
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	426 ``i18n`` --- ``'i'`` + 18 characters + ``'n'``) of software. Unicode solves
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	427 these problems by defining one code page for all scripts.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	428
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	429 If you want to include special characters in a string,
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	430 you can do so by using the Python Unicode-Escape encoding. The following
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	431 example shows how::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	432
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	433 >>> 'Hello\u0020World !'
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	434 'Hello World !'
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	435
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	436 The escape sequence ``\u0020`` indicates to insert the Unicode character with
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	437 the ordinal value 0x0020 (the space character) at the given position.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	438
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	439 Other characters are interpreted by using their respective ordinal values
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	440 directly as Unicode ordinals. If you have literal strings in the standard
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	441 Latin-1 encoding that is used in many Western countries, you will find it
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	442 convenient that the lower 256 characters of Unicode are the same as the 256
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	443 characters of Latin-1.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	444
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	445 Apart from these standard encodings, Python provides a whole set of other ways
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	446 of creating Unicode strings on the basis of a known encoding.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	447
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	448 To convert a string into a sequence of bytes using a specific encoding,
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	449 string objects provide an :func:`encode` method that takes one argument, the
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	450 name of the encoding. Lowercase names for encodings are preferred. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	451
43131 d15d9c6beda7 Fix unicode issue in tutorial. Georg Brandl <georg@python.org> parents: 43115 diff changeset	452 >>> "Äpfel".encode('utf-8')
d15d9c6beda7 Fix unicode issue in tutorial. Georg Brandl <georg@python.org> parents: 43115 diff changeset	453 b'\xc3\x84pfel'
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	454
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	455 .. _tut-lists:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	456
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	457 Lists
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	458 -----
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	459
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	460 Python knows a number of compound data types, used to group together other
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	461 values. The most versatile is the list, which can be written as a list of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	462 comma-separated values (items) between square brackets. List items need not all
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	463 have the same type. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	464
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	465 >>> a = ['spam', 'eggs', 100, 1234]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	466 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	467 ['spam', 'eggs', 100, 1234]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	468
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	469 Like string indices, list indices start at 0, and lists can be sliced,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	470 concatenated and so on::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	471
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	472 >>> a[0]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	473 'spam'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	474 >>> a[3]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	475 1234
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	476 >>> a[-2]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	477 100
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	478 >>> a[1:-1]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	479 ['eggs', 100]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	480 >>> a[:2] + ['bacon', 2*2]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	481 ['spam', 'eggs', 'bacon', 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	482 >>> 3*a[:3] + ['Boo!']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	483 ['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boo!']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	484
59876 90217a86642d Merged revisions 78966,78970,79018,79026-79027,79055,79156,79159,79163-79164,79173,79176,79194,79208,79212 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 56218 diff changeset	485 All slice operations return a new list containing the requested elements. This
90217a86642d Merged revisions 78966,78970,79018,79026-79027,79055,79156,79159,79163-79164,79173,79176,79194,79208,79212 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 56218 diff changeset	486 means that the following slice returns a shallow copy of the list a::
90217a86642d Merged revisions 78966,78970,79018,79026-79027,79055,79156,79159,79163-79164,79173,79176,79194,79208,79212 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 56218 diff changeset	487
90217a86642d Merged revisions 78966,78970,79018,79026-79027,79055,79156,79159,79163-79164,79173,79176,79194,79208,79212 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 56218 diff changeset	488 >>> a[:]
90217a86642d Merged revisions 78966,78970,79018,79026-79027,79055,79156,79159,79163-79164,79173,79176,79194,79208,79212 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 56218 diff changeset	489 ['spam', 'eggs', 100, 1234]
90217a86642d Merged revisions 78966,78970,79018,79026-79027,79055,79156,79159,79163-79164,79173,79176,79194,79208,79212 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 56218 diff changeset	490
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	491 Unlike strings, which are immutable, it is possible to change individual
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	492 elements of a list::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	493
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	494 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	495 ['spam', 'eggs', 100, 1234]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	496 >>> a[2] = a[2] + 23
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	497 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	498 ['spam', 'eggs', 123, 1234]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	499
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	500 Assignment to slices is also possible, and this can even change the size of the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	501 list or clear it entirely::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	502
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	503 >>> # Replace some items:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	504 ... a[0:2] = [1, 12]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	505 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	506 [1, 12, 123, 1234]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	507 >>> # Remove some:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	508 ... a[0:2] = []
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	509 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	510 [123, 1234]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	511 >>> # Insert some:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	512 ... a[1:1] = ['bletch', 'xyzzy']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	513 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	514 [123, 'bletch', 'xyzzy', 1234]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	515 >>> # Insert (a copy of) itself at the beginning
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	516 >>> a[:0] = a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	517 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	518 [123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	519 >>> # Clear the list: replace all items with an empty list
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	520 >>> a[:] = []
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	521 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	522 []
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	523
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	524 The built-in function :func:`len` also applies to lists::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	525
43815 85cf334b32c3 Merged revisions 58886-58929 via svnmerge from Guido van Rossum <guido@python.org> parents: 43449 diff changeset	526 >>> a = ['a', 'b', 'c', 'd']
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	527 >>> len(a)
43815 85cf334b32c3 Merged revisions 58886-58929 via svnmerge from Guido van Rossum <guido@python.org> parents: 43449 diff changeset	528 4
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	529
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	530 It is possible to nest lists (create lists containing other lists), for
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	531 example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	532
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	533 >>> q = [2, 3]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	534 >>> p = [1, q, 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	535 >>> len(p)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	536 3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	537 >>> p[1]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	538 [2, 3]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	539 >>> p[1][0]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	540 2
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	541
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	542 You can add something to the end of the list::
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	543
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43157 diff changeset	544 >>> p[1].append('xtra')
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	545 >>> p
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	546 [1, [2, 3, 'xtra'], 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	547 >>> q
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	548 [2, 3, 'xtra']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	549
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	550 Note that in the last example, ``p[1]`` and ``q`` really refer to the same
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	551 object! We'll come back to object semantics later.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	552
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	553
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	554 .. _tut-firststeps:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	555
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	556 First Steps Towards Programming
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	557 ===============================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	558
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	559 Of course, we can use Python for more complicated tasks than adding two and two
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	560 together. For instance, we can write an initial sub-sequence of the Fibonacci
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	561 series as follows::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	562
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	563 >>> # Fibonacci series:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	564 ... # the sum of two elements defines the next
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	565 ... a, b = 0, 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	566 >>> while b < 10:
44570 0aae14950e0b Fix merge glitch. Georg Brandl <georg@python.org> parents: 44568 diff changeset	567 ... print(b)
0aae14950e0b Fix merge glitch. Georg Brandl <georg@python.org> parents: 44568 diff changeset	568 ... a, b = b, a+b
50563 93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	569 ...
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	570 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	571 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	572 2
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	573 3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	574 5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	575 8
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	576
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	577 This example introduces several new features.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	578
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	579 * The first line contains a multiple assignment: the variables ``a`` and ``b``
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	580 simultaneously get the new values 0 and 1. On the last line this is used again,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	581 demonstrating that the expressions on the right-hand side are all evaluated
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	582 first before any of the assignments take place. The right-hand side expressions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	583 are evaluated from the left to the right.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	584
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	585 * The :keyword:`while` loop executes as long as the condition (here: ``b < 10``)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	586 remains true. In Python, like in C, any non-zero integer value is true; zero is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	587 false. The condition may also be a string or list value, in fact any sequence;
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	588 anything with a non-zero length is true, empty sequences are false. The test
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	589 used in the example is a simple comparison. The standard comparison operators
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	590 are written the same as in C: ``<`` (less than), ``>`` (greater than), ``==``
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	591 (equal to), ``<=`` (less than or equal to), ``>=`` (greater than or equal to)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	592 and ``!=`` (not equal to).
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	593
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	594 * The body of the loop is indented: indentation is Python's way of grouping
74178 f0197f1d158e I do not think we will ever have auto-indent at the prompt, that is for IPython and the like. Georg Brandl <georg@python.org> parents: 68257 diff changeset	595 statements. At the interactive prompt, you have to type a tab or space(s) for
f0197f1d158e I do not think we will ever have auto-indent at the prompt, that is for IPython and the like. Georg Brandl <georg@python.org> parents: 68257 diff changeset	596 each indented line. In practice you will prepare more complicated input
f0197f1d158e I do not think we will ever have auto-indent at the prompt, that is for IPython and the like. Georg Brandl <georg@python.org> parents: 68257 diff changeset	597 for Python with a text editor; all decent text editors have an auto-indent
f0197f1d158e I do not think we will ever have auto-indent at the prompt, that is for IPython and the like. Georg Brandl <georg@python.org> parents: 68257 diff changeset	598 facility. When a compound statement is entered interactively, it must be
f0197f1d158e I do not think we will ever have auto-indent at the prompt, that is for IPython and the like. Georg Brandl <georg@python.org> parents: 68257 diff changeset	599 followed by a blank line to indicate completion (since the parser cannot
f0197f1d158e I do not think we will ever have auto-indent at the prompt, that is for IPython and the like. Georg Brandl <georg@python.org> parents: 68257 diff changeset	600 guess when you have typed the last line). Note that each line within a basic
f0197f1d158e I do not think we will ever have auto-indent at the prompt, that is for IPython and the like. Georg Brandl <georg@python.org> parents: 68257 diff changeset	601 block must be indented by the same amount.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	602
81615 add66b51b890 expressions -> arguments Benjamin Peterson <benjamin@python.org> parents: 79681 diff changeset	603 * The :func:`print` function writes the value of the argument(s) it is given.
add66b51b890 expressions -> arguments Benjamin Peterson <benjamin@python.org> parents: 79681 diff changeset	604 It differs from just writing the expression you want to write (as we did
add66b51b890 expressions -> arguments Benjamin Peterson <benjamin@python.org> parents: 79681 diff changeset	605 earlier in the calculator examples) in the way it handles multiple arguments,
add66b51b890 expressions -> arguments Benjamin Peterson <benjamin@python.org> parents: 79681 diff changeset	606 floating point quantities, and strings. Strings are printed without quotes,
add66b51b890 expressions -> arguments Benjamin Peterson <benjamin@python.org> parents: 79681 diff changeset	607 and a space is inserted between items, so you can format things nicely, like
add66b51b890 expressions -> arguments Benjamin Peterson <benjamin@python.org> parents: 79681 diff changeset	608 this::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	609
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	610 >>> i = 256*256
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	611 >>> print('The value of i is', i)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	612 The value of i is 65536
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	613
81617 66a22b6a3414 end is a keyword argument Benjamin Peterson <benjamin@python.org> parents: 81615 diff changeset	614 The keyword argument end can be used to avoid the newline after the output,
66a22b6a3414 end is a keyword argument Benjamin Peterson <benjamin@python.org> parents: 81615 diff changeset	615 or end the output with a different string::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	616
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	617 >>> a, b = 0, 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	618 >>> while b < 1000:
66161 ec42112c2100 Backport rev86422 for issue 10304 from py3k to 31-maint Terry Reedy <tjreedy@udel.edu> parents: 66076 diff changeset	619 ... print(b, end=',')
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	620 ... a, b = b, a+b
50563 93cf3a6ead63 Merged revisions 68226 via svnmerge from Georg Brandl <georg@python.org> parents: 49180 diff changeset	621 ...
66161 ec42112c2100 Backport rev86422 for issue 10304 from py3k to 31-maint Terry Reedy <tjreedy@udel.edu> parents: 66076 diff changeset	622 1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/introduction.rst @ 83447:678564fc0962 3.3