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Diffstat (limited to 'Doc/reference/datamodel.rst')
| -rw-r--r-- | Doc/reference/datamodel.rst | 17 |
1 files changed, 13 insertions, 4 deletions
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 69ac9dc..1e85f83 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1289,8 +1289,7 @@ Basic customization instances are compared by object identity ("address"). See also the description of :meth:`__hash__` for some important notes on creating :term:`hashable` objects which support custom comparison operations and are - usable as dictionary keys. (Note: the restriction that exceptions are not - propagated by :meth:`__cmp__` has been removed since Python 1.5.) + usable as dictionary keys. .. method:: object.__hash__(self) @@ -1307,8 +1306,18 @@ Basic customization (e.g., using exclusive or) the hash values for the components of the object that also play a part in comparison of objects. - :meth:`__hash__` may also return a long integer object; the 32-bit integer is - then derived from the hash of that object. + If a class does not define a :meth:`__cmp__` or :meth:`__eq__` method it + should not define a :meth:`__hash__` operation either; if it defines + :meth:`__cmp__` or :meth:`__eq__` but not :meth:`__hash__`, its instances + will not be usable as dictionary keys. If a class defines mutable objects + and implements a :meth:`__cmp__` or :meth:`__eq__` method, it should not + implement :meth:`__hash__`, since the dictionary implementation requires that + a key's hash value is immutable (if the object's hash value changes, it will + be in the wrong hash bucket). + + User-defined classes have :meth:`__cmp__` and :meth:`__hash__` methods + by default; with them, all objects compare unequal and ``x.__hash__()`` + returns ``id(x)``. .. method:: object.__bool__(self) |