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945
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1 New features of classes
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2 =======================
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3
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4 A class can implement certain operations that are invoked by special
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5 syntax (such as subscription or arithmetic operations) by defining
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6 methods with special names.
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7
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8
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9 Special methods for any type
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10 ----------------------------
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11
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12 __repr__(self) --> string
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13
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14 Used by the print statement and conversions (reverse quotes) to
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15 compute the string representation of an object.
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16
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17 __cmp__(self, other) --> int
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18
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19 Used by all comparison operations. Should return -1 if self<other, 0
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20 if self==other, +1 if self>other. Due to limitations in the
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21 interpreter, exceptions raised by comparisons are ignored, and the
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22 objects will be considered equal in this case.
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23
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24
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25 Special methods for sequence and mapping types
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26 ----------------------------------------------
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27
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28 __len__(self) --> int
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29
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30 Used by the built-in function len(). Should return the length of the
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31 object, which should be >= 0. Also, an object whose __len__() method
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32 returns 0
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33
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34 __getitem__(self, key) --> value
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35
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36 Used to implement value = self[key]. Note that the special
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37 interpretation of negative keys (if the class wishes to emulate a
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38 sequence type) is up to the __getitem__ method.
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39
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40 __setitem__(self, key, value)
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41
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42 Used to implement self[key] = value. Same note as for __getitem__.
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43
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44 __delitem__(self, key)
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45
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46 Used to implement del self[key]. Same note as for __getitem__.
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47
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48
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49 Special methods for sequence types
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50 ----------------------------------
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51
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52 __getslice__(self, i, j) --> sequence
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53
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54 Used to implement self[i:j]. Note that missing i or j are replaced by
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55 0 or len(self), respectively, and len(self) has been added to negative
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56 i or j.
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57
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58 __setslice__(self, i, j, sequence)
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59
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60 Used to implement self[i:j] = value. Same note as for __getslice__.
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61
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62 __delslice__(self, i, j)
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63
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64 Used to implement del self[i:j]. Same note as for __getslice__.
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65
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66
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67 Special methods for numeric types
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68 ---------------------------------
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69
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70 __add__, __sub__, __mul__, __div__, __mod__, __divmod__, __pow__,
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71 __lshift__, __rshift__, __and__, __xor__, __or__
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72
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73 Used to implement the binary arithmetic operations (divmod and pow are
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74 called by built-in functions). All have the call pattern
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75 func(self, other) --> number.
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76
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77 __neg__, __pos__, __abs__, __invert__
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78
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79 Used to implement the unary arithmetic operations (-, +, abs and ~).
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80 All have the call pattern func(self) --> number.
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81
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82 __nonzero__(self) --> int
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83
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84 Used to implement boolean testing. An alternative name for this
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85 method is __len__.
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86
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87 __coerce__(self, other) --> (self1, other1) or None
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88
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89 Used to implement "mixed-mode" numeric arithmetic. Either return a
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90 tuple containing self and other converted to some common type, or None
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91 if no way of conversion is known. When the common type would be the
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92 type of other, it is sufficient to return None, since the interpreter
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93 will also ask the other object to attempt a coercion (but sometimes,
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94 if the implementation of the other type cannot be changed, it is
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95 useful to do the conversion to the other type here).
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96
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97 __int__(self) --> int
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98 __long__(self) --> long
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99 __float__(self) --> float
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100
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101 Used to implement the built-in functions int(), long() and float().
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102
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103
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104 Notes
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105 -----
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106
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107 Except for __repr__ and __cmp__, when no appropriate method is
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108 defined, attempts to execute the operation raise an exception. For
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109 __repr__ and __cmp__, the traditional interpretations are used
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110 in this case.
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