x.__del__()-- the former decrements the reference count for
xby 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_tracebackkeeps 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_tracebackkeeps 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
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
sys.stderrinstead. 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.
This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.
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,
Falseis used for false and
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.
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.)
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.
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