Python in a Nutshell

Name

__hash__

Synopsis

__hash__(self)

The hash( x ) built-in function call, and using x as a dictionary key (typically, D [ x ] where D is a dictionary), call x .__hash__( ). __hash__ must return a 32-bit int such that x == y implies hash( x )==hash( y ), and must always return the same value for a given object.

When __hash__ is absent, hash( x ) and using x as a dictionary key call id( x ) instead, as long as __cmp__ and __eq__ are also absent.

Any x such that hash( x ) returns a result, rather than raising an exception, is known as a hashable object. When __hash__ is absent, but __cmp__ or __eq__ is present, hash( x ) and using x as a dictionary key raise an exception. In this case, x is not hashable and cannot be a dictionary key.

You normally define __hash__ only for immutable objects that also define __cmp__ and/or __eq__. Note that, if there exists any y such that x == y, even if y is of a different type, and both x and y are hashable, you must ensure that hash( x )==hash( y ).