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Basic Special Methods

In addition to __init__ and __str__ there are a number of methods which are appropriate for classes of all kinds.

__init__ ( self , args... )

Called when a new instance of the is created. Not that this overrides any superclass __init__ method; to to superclass initialization first, you must evaluate the superclass __init__ like this: super( Class ,self).__init__( args... ). The super function identifies the superclass of your class, Class .

__del__ ( self )

Called when the this object is no longer referenced anywhere in the running program; the object is about to be removed by garbage collection. This is rarely used. Note that this is called as part of Python garbage collection; it is not called by the del statement.

__repr__ ( self ) → String

Called by the repr built-in function. Typically, the string returned by this will look like a valid Python expression to reconstruct the object.

__str__ ( self ) → String

Called by the str built-in function. This is called implicitly by the print statement to convert an object to a convenient, “pretty” string representation.

__cmp__ ( self , other ) → int

Called by all comparison operations, including the cmp function. This must return a negative integer if self < other, zero if self == other, a positive integer if self > other. If no __cmp__ operation is defined, class instances are compared by object identity.

In addition to the basic __cmp__, there are additional methods which can be used to define the behaviors of individual comparison operators. These include __lt__, __le__, __eq__, __ne__, __gt__, and __ge__ which match the operators <, <=, ==, !=, > and >=.

__hash__ ( self ) → int

Called during dictionary operations, and by the built-in function hash to transform an object to a unique 32-bit integer hash value. Objects which compare equal (__cmp__ returns 0) should also have the same hash value. If a class does not define a __cmp__ method it should not define a __hash__ operation either. Classes with mutable objects can define __cmp__ but should not define __hash__, or objects would move around in the dictionary.

__nonzero__ ( self ) → boolean

Called during truth value testing; must return 0 or 1. If this method is not defined, and __len__ is defined, then __len__ is called based on the assumption that this is a collection. If neither function is defined, all values are considered True.

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