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
).
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