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Python Reference Manual

Previous: 3.3 Special method names Up: 3.3 Special method names Next: 3.3.2 Customizing attribute access

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3.3.1 Basic customization

__init__(

self[, ...])

Called when the instance is created. The arguments are those passed to the class constructor expression. If a base class has an __init__() method, the derived class's __init__() method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: "BaseClass.__init__(self, [args...])". As a special constraint on constructors, no value may be returned; doing so will cause a TypeError to be raised at runtime.

__del__(

self)

Called when the instance is about to be destroyed. This is also called a destructor. If a base class has a __del__() method, the derived class's __del__() method, if any, must explicitly call it to ensure proper deletion of the base class part of the instance. Note that it is possible (though not recommended!) for the __del__() method to postpone destruction of the instance by creating a new reference to it. It may then be called at a later time when this new reference is deleted. It is not guaranteed that __del__() methods are called for objects that still exist when the interpreter exits.

Note: "del x" doesn't directly call x.__del__() -- the former decrements the reference count for x by one, and the latter is only called when x's reference count reaches zero. Some common situations that may prevent the reference count of an object from going to zero include: circular references between objects (e.g., a doubly-linked list or a tree data structure with parent and child pointers); a reference to the object on the stack frame of a function that caught an exception (the traceback stored in sys.exc_traceback keeps the stack frame alive); or a reference to the object on the stack frame that raised an unhandled exception in interactive mode (the traceback stored in sys.last_traceback keeps the stack frame alive). The first situation can only be remedied by explicitly breaking the cycles; the latter two situations can be resolved by storing None in sys.exc_traceback or sys.last_traceback. Circular references which are garbage are detected when the option cycle detector is enabled (it's on by default), but can only be cleaned up if there are no Python-level __del__() methods involved. Refer to the documentation for the gc module for more information about how __del__() methods are handled by the cycle detector, particularly the description of the garbage value.

Warning: Due to the precarious circumstances under which __del__() methods are invoked, exceptions that occur during their execution are ignored, and a warning is printed to sys.stderr instead. Also, when __del__() is invoked in response to a module being deleted (e.g., when execution of the program is done), other globals referenced by the __del__() method may already have been deleted. For this reason, __del__() methods should do the absolute minimum needed to maintain external invariants. Starting with version 1.5, Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted; if no other references to such globals exist, this may help in assuring that imported modules are still available at the time when the __del__() method is called.

__repr__(

self)

Called by the repr() built-in function and by string conversions (reverse quotes) to compute the ``official'' string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form "<...some useful description...>" should be returned. The return value must be a string object. If a class defines __repr__() but not __str__(), then __repr__() is also used when an ``informal'' string representation of instances of that class is required.

This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.

__str__(

self)

Called by the str() built-in function and by the print statement to compute the ``informal'' string representation of an object. This differs from __repr__() in that it does not have to be a valid Python expression: a more convenient or concise representation may be used instead. The return value must be a string object.

__lt__(

self, other)

__le__(

self, other)

__eq__(

self, other)

__ne__(

self, other)

__gt__(

self, other)

__ge__(

self, other)

New in version 2.1. These are the so-called ``rich comparison'' methods, and are called for comparison operators in preference to __cmp__() below. The correspondence between operator symbols and method names is as follows: x<y calls x.__lt__(y), x<=y calls x.__le__(y), x==y calls x.__eq__(y), x!=y and x<>y call x.__ne__(y), x>y calls x.__gt__(y), and x>=y calls x.__ge__(y). These methods can return any value, but if the comparison operator is used in a Boolean context, the return value should be interpretable as a Boolean value, else a TypeError will be raised. By convention, False is used for false and True for true.

There are no implied relationships among the comparison operators. The truth of x==y does not imply that x!=y is false. Accordingly, when defining __eq__, one should also define __ne__ so that the operators will behave as expected.

There are no reflected (swapped-argument) versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, __lt__() and __gt__() are each other's reflection, __le__() and __ge__() are each other's reflection, and __eq__() and __ne__() are their own reflection.

Arguments to rich comparison methods are never coerced. A rich comparison method may return NotImplemented if it does not implement the operation for a given pair of arguments.

__cmp__(

self, other)

Called by comparison operations if rich comparison (see above) is not defined. Should return a negative integer if self < other, zero if self == other, a positive integer if self > other. If no __cmp__(), __eq__() or __ne__() operation is defined, class instances are compared by object identity (``address''). See also the description of __hash__() for some important notes on creating objects which support custom comparison operations and are usable as dictionary keys. (Note: the restriction that exceptions are not propagated by __cmp__() has been removed since Python 1.5.)

__rcmp__(

self, other)

Changed in version 2.1: No longer supported.

__hash__(

self)

Called for the key object for dictionaryoperations, and by the built-in function hash(). Should return a 32-bit integer usable as a hash value for dictionary operations. The only required property is that objects which compare equal have the same hash value; it is advised to somehow mix together (e.g., using exclusive or) the hash values for the components of the object that also play a part in comparison of objects. If a class does not define a __cmp__() method it should not define a __hash__() operation either; if it defines __cmp__() or __eq__() but not __hash__(), its instances will not be usable as dictionary keys. If a class defines mutable objects and implements a __cmp__() or __eq__() method, it should not implement __hash__(), since the dictionary implementation requires that a key's hash value is immutable (if the object's hash value changes, it will be in the wrong hash bucket).

__nonzero__(

self)

Called to implement truth value testing, and the built-in operation bool(); should return False or True, or their integer equivalents 0 or 1. When this method is not defined, __len__() is called, if it is defined (see below). If a class defines neither __len__() nor __nonzero__(), all its instances are considered true.

__unicode__(

self)

Called to implement unicode() builtin; should return a Unicode object. When this method is not defined, string conversion is attempted, and the result of string conversion is converted to Unicode using the system default encoding.

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Python Reference Manual

Previous: 3.3 Special method names Up: 3.3 Special method names Next: 3.3.2 Customizing attribute access

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Release 2.3.4, documentation updated on May 20, 2004.