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Describe the default hash correctly, and mark a couple of CPython implementation details

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Nick Coghlan 2012-05-20 18:30:49 +10:00
parent 78770f0b78
commit 337b2bfbc4
1 changed files with 16 additions and 4 deletions
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Doc/reference/datamodel.rst
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@ -35,12 +35,19 @@ represented by objects.)
Every object has an identity, a type and a value. An object's *identity* never Every object has an identity, a type and a value. An object's *identity* never
changes once it has been created; you may think of it as the object's address in changes once it has been created; you may think of it as the object's address in
memory. The ':keyword:`is`' operator compares the identity of two objects; the memory. The ':keyword:`is`' operator compares the identity of two objects; the
:func:`id` function returns an integer representing its identity (currently :func:`id` function returns an integer representing its identity.
implemented as its address). An object's :dfn:`type` is also unchangeable. [#]_
.. impl-detail::
For CPython, ``id(x)`` is the memory address where ``x`` is stored.
An object's type determines the operations that the object supports (e.g., "does An object's type determines the operations that the object supports (e.g., "does
it have a length?") and also defines the possible values for objects of that it have a length?") and also defines the possible values for objects of that
type. The :func:`type` function returns an object's type (which is an object type. The :func:`type` function returns an object's type (which is an object
itself). The *value* of some objects can change. Objects whose value can itself). Like its identity, an object's :dfn:`type` is also unchangeable.
[#]_
The *value* of some objects can change. Objects whose value can
change are said to be *mutable*; objects whose value is unchangeable once they change are said to be *mutable*; objects whose value is unchangeable once they
are created are called *immutable*. (The value of an immutable container object are created are called *immutable*. (The value of an immutable container object
that contains a reference to a mutable object can change when the latter's value that contains a reference to a mutable object can change when the latter's value
@ -1258,7 +1265,12 @@ Basic customization
User-defined classes have :meth:`__eq__` and :meth:`__hash__` methods User-defined classes have :meth:`__eq__` and :meth:`__hash__` methods
by default; with them, all objects compare unequal (except with themselves) by default; with them, all objects compare unequal (except with themselves)
and ``x.__hash__()`` returns ``id(x)``. and ``x.__hash__()`` returns an appropriate value such that ``x == y``
implies both that ``x is y`` and ``hash(x) == hash(y)``.
.. impl-detail::
CPython uses ``hash(id(x))`` as the default hash for class instances.
Classes which inherit a :meth:`__hash__` method from a parent class but Classes which inherit a :meth:`__hash__` method from a parent class but
change the meaning of :meth:`__eq__` such that the hash value returned is no change the meaning of :meth:`__eq__` such that the hash value returned is no