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Document how to use Set and MutableSet as a mixin.

This commit is contained in:
Raymond Hettinger 2008-02-09 03:25:08 +00:00
parent 74b6495b34
commit 7aebb64bab
2 changed files with 20 additions and 3 deletions
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19
Doc/library/collections.rst
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@ -58,7 +58,7 @@ ABC Inherits Abstract Methods Mixin M
``insert``, ``remove``, and ``__iadd__``
and ``__len__``
:class:`Set` :class:`Sized`, ``__len__``, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``,
:class:`Set` \(1) \(2) :class:`Sized`, ``__len__``, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``,
:class:`Iterable`, ``__iter__``, and ``__gt__``, ``__ge__``, ``__and__``, ``__or__``
:class:`Container` ``__contains__`` ``__sub__``, ``__xor__``, and ``isdisjoint``
@ -100,6 +100,23 @@ The ABC supplies the remaining methods such as :meth:`__and__` and
s2 = ListBasedSet('defghi')
overlap = s1 & s2 # The __and__() method is supported automatically
Notes on using :class:`Set` and :class:`MutableSet` as a mixin:
(1)
Since some set operations create new sets, the default mixin methods need
a way to create new instances from an iterable. The class constructor is
assumed to have a signature in the form ``ClassName(iterable)``.
That assumption is factored-out to a singleinternal classmethod called
:meth:`_from_iterable` which calls ``cls(iterable)`` to produce a new set.
If the :class:`Set` mixin is being used in a class with a different
constructor signature, you will need to override :meth:`from_iterable`
with a classmethod that can construct new instances from
an iterable argument.
(2)
To override the comparisons (presumably for speed, as the
semantics are fixed), redefine :meth:`__le__` and
then the other operations will automatically follow suit.
(For more about ABCs, see the :mod:`abc` module and :pep:`3119`.)

4
Lib/_abcoll.py
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@ -207,9 +207,9 @@ class Set(Sized, Iterable, Container):
'''Construct an instance of the class from any iterable input.
Must override this method if the class constructor signature
will not accept a frozenset for an input.
does not accept an iterable for an input.
'''
return cls(frozenset(it))
return cls(it)
def __and__(self, other):
if not isinstance(other, Iterable):