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

annotate Doc/tutorial/errors.rst @ 55434:d98b8a6ec2b8 legacy-trunk

#6564: fix section about the two raise syntaxes.

author	Georg Brandl <georg@python.org>
date	Fri, 24 Jul 2009 20:09:46 +0000
parents	acc07866b233
children	20bfc1d35989

rev	line source
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	1 .. _tut-errors:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	2
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	3 *********************
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	4 Errors and Exceptions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	5 *********************
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	6
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	7 Until now error messages haven't been more than mentioned, but if you have tried
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	8 out the examples you have probably seen some. There are (at least) two
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	9 distinguishable kinds of errors: syntax errors and exceptions.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	10
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	11
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	12 .. _tut-syntaxerrors:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	13
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	14 Syntax Errors
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	15 =============
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	16
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	17 Syntax errors, also known as parsing errors, are perhaps the most common kind of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	18 complaint you get while you are still learning Python::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	19
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	20 >>> while True print 'Hello world'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	21 File "<stdin>", line 1, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	22 while True print 'Hello world'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	23 ^
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	24 SyntaxError: invalid syntax
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	25
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	26 The parser repeats the offending line and displays a little 'arrow' pointing at
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	27 the earliest point in the line where the error was detected. The error is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	28 caused by (or at least detected at) the token preceding the arrow: in the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	29 example, the error is detected at the keyword :keyword:`print`, since a colon
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	30 (``':'``) is missing before it. File name and line number are printed so you
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	31 know where to look in case the input came from a script.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	32
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	33
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	34 .. _tut-exceptions:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	35
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	36 Exceptions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	37 ==========
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	38
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	39 Even if a statement or expression is syntactically correct, it may cause an
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	40 error when an attempt is made to execute it. Errors detected during execution
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	41 are called exceptions and are not unconditionally fatal: you will soon learn
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	42 how to handle them in Python programs. Most exceptions are not handled by
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	43 programs, however, and result in error messages as shown here::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	44
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	45 >>> 10 * (1/0)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	46 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	47 File "<stdin>", line 1, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	48 ZeroDivisionError: integer division or modulo by zero
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	49 >>> 4 + spam*3
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	50 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	51 File "<stdin>", line 1, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	52 NameError: name 'spam' is not defined
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	53 >>> '2' + 2
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	54 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	55 File "<stdin>", line 1, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	56 TypeError: cannot concatenate 'str' and 'int' objects
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	57
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	58 The last line of the error message indicates what happened. Exceptions come in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	59 different types, and the type is printed as part of the message: the types in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	60 the example are :exc:`ZeroDivisionError`, :exc:`NameError` and :exc:`TypeError`.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	61 The string printed as the exception type is the name of the built-in exception
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	62 that occurred. This is true for all built-in exceptions, but need not be true
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	63 for user-defined exceptions (although it is a useful convention). Standard
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	64 exception names are built-in identifiers (not reserved keywords).
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	65
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	66 The rest of the line provides detail based on the type of exception and what
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	67 caused it.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	68
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	69 The preceding part of the error message shows the context where the exception
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	70 happened, in the form of a stack traceback. In general it contains a stack
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	71 traceback listing source lines; however, it will not display lines read from
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	72 standard input.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	73
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	74 :ref:`bltin-exceptions` lists the built-in exceptions and their meanings.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	75
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	76
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	77 .. _tut-handling:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	78
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	79 Handling Exceptions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	80 ===================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	81
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	82 It is possible to write programs that handle selected exceptions. Look at the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	83 following example, which asks the user for input until a valid integer has been
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	84 entered, but allows the user to interrupt the program (using :kbd:`Control-C` or
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	85 whatever the operating system supports); note that a user-generated interruption
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	86 is signalled by raising the :exc:`KeyboardInterrupt` exception. ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	87
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	88 >>> while True:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	89 ... try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	90 ... x = int(raw_input("Please enter a number: "))
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	91 ... break
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	92 ... except ValueError:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	93 ... print "Oops! That was no valid number. Try again..."
50522 ee9ced92843a Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from Georg Brandl <georg@python.org> parents: 49207 diff changeset	94 ...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	95
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	96 The :keyword:`try` statement works as follows.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	97
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	98 * First, the try clause (the statement(s) between the :keyword:`try` and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	99 :keyword:`except` keywords) is executed.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	100
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	101 * If no exception occurs, the except clause is skipped and execution of the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	102 :keyword:`try` statement is finished.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	103
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	104 * If an exception occurs during execution of the try clause, the rest of the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	105 clause is skipped. Then if its type matches the exception named after the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	106 :keyword:`except` keyword, the except clause is executed, and then execution
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	107 continues after the :keyword:`try` statement.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	108
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	109 * If an exception occurs which does not match the exception named in the except
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	110 clause, it is passed on to outer :keyword:`try` statements; if no handler is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	111 found, it is an unhandled exception and execution stops with a message as
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	112 shown above.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	113
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	114 A :keyword:`try` statement may have more than one except clause, to specify
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	115 handlers for different exceptions. At most one handler will be executed.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	116 Handlers only handle exceptions that occur in the corresponding try clause, not
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	117 in other handlers of the same :keyword:`try` statement. An except clause may
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	118 name multiple exceptions as a parenthesized tuple, for example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	119
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	120 ... except (RuntimeError, TypeError, NameError):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	121 ... pass
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	122
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	123 The last except clause may omit the exception name(s), to serve as a wildcard.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	124 Use this with extreme caution, since it is easy to mask a real programming error
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	125 in this way! It can also be used to print an error message and then re-raise
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	126 the exception (allowing a caller to handle the exception as well)::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	127
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	128 import sys
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	129
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	130 try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131 f = open('myfile.txt')
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	132 s = f.readline()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	133 i = int(s.strip())
45048 250f29bab61b Demonstrate new except/as syntax. Raymond Hettinger <python@rcn.com> parents: 44315 diff changeset	134 except IOError as (errno, strerror):
47282 70f9a5db279c update the tutorial to use str.format Benjamin Peterson <benjamin@python.org> parents: 45048 diff changeset	135 print "I/O error({0}): {1}".format(errno, strerror)
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	136 except ValueError:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	137 print "Could not convert data to an integer."
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	138 except:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	139 print "Unexpected error:", sys.exc_info()[0]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	140 raise
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	141
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	142 The :keyword:`try` ... :keyword:`except` statement has an optional *else
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	143 clause*, which, when present, must follow all except clauses. It is useful for
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	144 code that must be executed if the try clause does not raise an exception. For
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	145 example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	146
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	147 for arg in sys.argv[1:]:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	148 try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	149 f = open(arg, 'r')
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	150 except IOError:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	151 print 'cannot open', arg
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	152 else:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	153 print arg, 'has', len(f.readlines()), 'lines'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	154 f.close()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	155
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	156 The use of the :keyword:`else` clause is better than adding additional code to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	157 the :keyword:`try` clause because it avoids accidentally catching an exception
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	158 that wasn't raised by the code being protected by the :keyword:`try` ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	159 :keyword:`except` statement.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	160
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	161 When an exception occurs, it may have an associated value, also known as the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	162 exception's argument. The presence and type of the argument depend on the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	163 exception type.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	164
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	165 The except clause may specify a variable after the exception name (or tuple).
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	166 The variable is bound to an exception instance with the arguments stored in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	167 ``instance.args``. For convenience, the exception instance defines
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	168 :meth:`__getitem__` and :meth:`__str__` so the arguments can be accessed or
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	169 printed directly without having to reference ``.args``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	170
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	171 But use of ``.args`` is discouraged. Instead, the preferred use is to pass a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	172 single argument to an exception (which can be a tuple if multiple arguments are
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	173 needed) and have it bound to the ``message`` attribute. One may also
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	174 instantiate an exception first before raising it and add any attributes to it as
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	175 desired. ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	176
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	177 >>> try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	178 ... raise Exception('spam', 'eggs')
45048 250f29bab61b Demonstrate new except/as syntax. Raymond Hettinger <python@rcn.com> parents: 44315 diff changeset	179 ... except Exception as inst:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	180 ... print type(inst) # the exception instance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	181 ... print inst.args # arguments stored in .args
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	182 ... print inst # __str__ allows args to printed directly
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	183 ... x, y = inst # __getitem__ allows args to be unpacked directly
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	184 ... print 'x =', x
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	185 ... print 'y =', y
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	186 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	187 <type 'exceptions.Exception'>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	188 ('spam', 'eggs')
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	189 ('spam', 'eggs')
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	190 x = spam
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	191 y = eggs
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	192
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	193 If an exception has an argument, it is printed as the last part ('detail') of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	194 the message for unhandled exceptions.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	195
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	196 Exception handlers don't just handle exceptions if they occur immediately in the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	197 try clause, but also if they occur inside functions that are called (even
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	198 indirectly) in the try clause. For example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	199
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	200 >>> def this_fails():
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	201 ... x = 1/0
50522 ee9ced92843a Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from Georg Brandl <georg@python.org> parents: 49207 diff changeset	202 ...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	203 >>> try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	204 ... this_fails()
45048 250f29bab61b Demonstrate new except/as syntax. Raymond Hettinger <python@rcn.com> parents: 44315 diff changeset	205 ... except ZeroDivisionError as detail:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	206 ... print 'Handling run-time error:', detail
50522 ee9ced92843a Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from Georg Brandl <georg@python.org> parents: 49207 diff changeset	207 ...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	208 Handling run-time error: integer division or modulo by zero
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	209
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	210
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	211 .. _tut-raising:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	212
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	213 Raising Exceptions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	214 ==================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	215
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	216 The :keyword:`raise` statement allows the programmer to force a specified
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	217 exception to occur. For example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	218
54350 acc07866b233 Merged revisions 72319-72320,72467,72661,72675-72679,72703,72708,72710,72712,72801-72802,72820,72822,72824,72826-72828,72830 via svnmerge from Georg Brandl <georg@python.org> parents: 50522 diff changeset	219 >>> raise NameError('HiThere')
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	220 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	221 File "<stdin>", line 1, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	222 NameError: HiThere
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	223
55434 d98b8a6ec2b8 #6564: fix section about the two raise syntaxes. Georg Brandl <georg@python.org> parents: 54350 diff changeset	224 The argument to :keyword:`raise` is an exception class or instance to be
d98b8a6ec2b8 #6564: fix section about the two raise syntaxes. Georg Brandl <georg@python.org> parents: 54350 diff changeset	225 raised. There is a deprecated alternate syntax that separates class and
d98b8a6ec2b8 #6564: fix section about the two raise syntaxes. Georg Brandl <georg@python.org> parents: 54350 diff changeset	226 constructor arguments; the above could be written as ``raise NameError,
d98b8a6ec2b8 #6564: fix section about the two raise syntaxes. Georg Brandl <georg@python.org> parents: 54350 diff changeset	227 'HiThere'``. Since it once was the only one available, the latter form is
d98b8a6ec2b8 #6564: fix section about the two raise syntaxes. Georg Brandl <georg@python.org> parents: 54350 diff changeset	228 prevalent in older code.
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	229
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	230 If you need to determine whether an exception was raised but don't intend to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	231 handle it, a simpler form of the :keyword:`raise` statement allows you to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	232 re-raise the exception::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	233
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	234 >>> try:
54350 acc07866b233 Merged revisions 72319-72320,72467,72661,72675-72679,72703,72708,72710,72712,72801-72802,72820,72822,72824,72826-72828,72830 via svnmerge from Georg Brandl <georg@python.org> parents: 50522 diff changeset	235 ... raise NameError('HiThere')
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	236 ... except NameError:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	237 ... print 'An exception flew by!'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	238 ... raise
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	239 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	240 An exception flew by!
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	241 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	242 File "<stdin>", line 2, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	243 NameError: HiThere
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	244
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	245
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	246 .. _tut-userexceptions:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	247
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	248 User-defined Exceptions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	249 =======================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	250
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	251 Programs may name their own exceptions by creating a new exception class.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	252 Exceptions should typically be derived from the :exc:`Exception` class, either
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	253 directly or indirectly. For example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	254
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	255 >>> class MyError(Exception):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	256 ... def __init__(self, value):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	257 ... self.value = value
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	258 ... def __str__(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	259 ... return repr(self.value)
50522 ee9ced92843a Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from Georg Brandl <georg@python.org> parents: 49207 diff changeset	260 ...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	261 >>> try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	262 ... raise MyError(2*2)
45048 250f29bab61b Demonstrate new except/as syntax. Raymond Hettinger <python@rcn.com> parents: 44315 diff changeset	263 ... except MyError as e:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	264 ... print 'My exception occurred, value:', e.value
50522 ee9ced92843a Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from Georg Brandl <georg@python.org> parents: 49207 diff changeset	265 ...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	266 My exception occurred, value: 4
54350 acc07866b233 Merged revisions 72319-72320,72467,72661,72675-72679,72703,72708,72710,72712,72801-72802,72820,72822,72824,72826-72828,72830 via svnmerge from Georg Brandl <georg@python.org> parents: 50522 diff changeset	267 >>> raise MyError('oops!')
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	268 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	269 File "<stdin>", line 1, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	270 __main__.MyError: 'oops!'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	271
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	272 In this example, the default :meth:`__init__` of :class:`Exception` has been
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	273 overridden. The new behavior simply creates the value attribute. This
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	274 replaces the default behavior of creating the args attribute.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	275
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	276 Exception classes can be defined which do anything any other class can do, but
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	277 are usually kept simple, often only offering a number of attributes that allow
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	278 information about the error to be extracted by handlers for the exception. When
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	279 creating a module that can raise several distinct errors, a common practice is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	280 to create a base class for exceptions defined by that module, and subclass that
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	281 to create specific exception classes for different error conditions::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	282
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	283 class Error(Exception):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	284 """Base class for exceptions in this module."""
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	285 pass
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	286
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	287 class InputError(Error):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	288 """Exception raised for errors in the input.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	289
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	290 Attributes:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	291 expression -- input expression in which the error occurred
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	292 message -- explanation of the error
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	293 """
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	294
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	295 def __init__(self, expression, message):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	296 self.expression = expression
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	297 self.message = message
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	298
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	299 class TransitionError(Error):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	300 """Raised when an operation attempts a state transition that's not
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	301 allowed.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	302
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	303 Attributes:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	304 previous -- state at beginning of transition
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	305 next -- attempted new state
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	306 message -- explanation of why the specific transition is not allowed
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	307 """
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	308
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	309 def __init__(self, previous, next, message):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	310 self.previous = previous
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	311 self.next = next
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	312 self.message = message
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	313
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	314 Most exceptions are defined with names that end in "Error," similar to the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	315 naming of the standard exceptions.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	316
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	317 Many standard modules define their own exceptions to report errors that may
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	318 occur in functions they define. More information on classes is presented in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	319 chapter :ref:`tut-classes`.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	320
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	321
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	322 .. _tut-cleanup:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	323
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	324 Defining Clean-up Actions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	325 =========================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	326
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	327 The :keyword:`try` statement has another optional clause which is intended to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	328 define clean-up actions that must be executed under all circumstances. For
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	329 example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	330
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	331 >>> try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	332 ... raise KeyboardInterrupt
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	333 ... finally:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	334 ... print 'Goodbye, world!'
50522 ee9ced92843a Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from Georg Brandl <georg@python.org> parents: 49207 diff changeset	335 ...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	336 Goodbye, world!
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	337 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	338 File "<stdin>", line 2, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	339 KeyboardInterrupt
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	340
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	341 A finally clause is always executed before leaving the :keyword:`try`
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	342 statement, whether an exception has occurred or not. When an exception has
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	343 occurred in the :keyword:`try` clause and has not been handled by an
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	344 :keyword:`except` clause (or it has occurred in a :keyword:`except` or
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	345 :keyword:`else` clause), it is re-raised after the :keyword:`finally` clause has
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	346 been executed. The :keyword:`finally` clause is also executed "on the way out"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	347 when any other clause of the :keyword:`try` statement is left via a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	348 :keyword:`break`, :keyword:`continue` or :keyword:`return` statement. A more
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	349 complicated example (having :keyword:`except` and :keyword:`finally` clauses in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	350 the same :keyword:`try` statement works as of Python 2.5)::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	351
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	352 >>> def divide(x, y):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	353 ... try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	354 ... result = x / y
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	355 ... except ZeroDivisionError:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	356 ... print "division by zero!"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	357 ... else:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	358 ... print "result is", result
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	359 ... finally:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	360 ... print "executing finally clause"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	361 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	362 >>> divide(2, 1)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	363 result is 2
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	364 executing finally clause
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	365 >>> divide(2, 0)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	366 division by zero!
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	367 executing finally clause
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	368 >>> divide("2", "1")
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	369 executing finally clause
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	370 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	371 File "<stdin>", line 1, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	372 File "<stdin>", line 3, in divide
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	373 TypeError: unsupported operand type(s) for /: 'str' and 'str'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	374
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	375 As you can see, the :keyword:`finally` clause is executed in any event. The
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	376 :exc:`TypeError` raised by dividing two strings is not handled by the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	377 :keyword:`except` clause and therefore re-raised after the :keyword:`finally`
49207 d17cc4d1766c Fix typo. Georg Brandl <georg@python.org> parents: 47282 diff changeset	378 clause has been executed.
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	379
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	380 In real world applications, the :keyword:`finally` clause is useful for
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	381 releasing external resources (such as files or network connections), regardless
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	382 of whether the use of the resource was successful.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	383
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	384
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	385 .. _tut-cleanup-with:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	386
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	387 Predefined Clean-up Actions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	388 ===========================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	389
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	390 Some objects define standard clean-up actions to be undertaken when the object
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	391 is no longer needed, regardless of whether or not the operation using the object
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	392 succeeded or failed. Look at the following example, which tries to open a file
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	393 and print its contents to the screen. ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	394
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	395 for line in open("myfile.txt"):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	396 print line
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	397
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	398 The problem with this code is that it leaves the file open for an indeterminate
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	399 amount of time after the code has finished executing. This is not an issue in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	400 simple scripts, but can be a problem for larger applications. The
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	401 :keyword:`with` statement allows objects like files to be used in a way that
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	402 ensures they are always cleaned up promptly and correctly. ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	403
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	404 with open("myfile.txt") as f:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	405 for line in f:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	406 print line
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	407
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	408 After the statement is executed, the file f is always closed, even if a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	409 problem was encountered while processing the lines. Other objects which provide
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	410 predefined clean-up actions will indicate this in their documentation.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	411
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	412

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/errors.rst @ 55434:d98b8a6ec2b8 legacy-trunk