According to Wikipedia:
A mixin is a class that contains methods for use by other classes without having to be the parent class of those other classes.
Taking the following class defined in Werkzeug
class ImmutableDictMixin(object):
"""Makes a :class:`dict` immutable.
.. versionadded:: 0.5
:private:
"""
_hash_cache = None
@classmethod
def fromkeys(cls, keys, value=None):
instance = super(cls, cls).__new__(cls)
instance.__init__(zip(keys, repeat(value)))
return instance
def __reduce_ex__(self, protocol):
return type(self), (dict(self),)
def _iter_hashitems(self):
return iteritems(self)
def __hash__(self):
if self._hash_cache is not None:
return self._hash_cache
rv = self._hash_cache = hash(frozenset(self._iter_hashitems()))
return rv
def setdefault(self, key, default=None):
is_immutable(self)
def update(self, *args, **kwargs):
is_immutable(self)
def pop(self, key, default=None):
is_immutable(self)
def popitem(self):
is_immutable(self)
def __setitem__(self, key, value):
is_immutable(self)
def __delitem__(self, key):
is_immutable(self)
def clear(self):
is_immutable(self)
then the following:
class ImmutableDict(ImmutableDictMixin, dict):
"""An immutable :class:`dict`.
.. versionadded:: 0.5
"""
def __repr__(self):
return '%s(%s)' % (
self.__class__.__name__,
dict.__repr__(self),
)
def copy(self):
"""Return a shallow mutable copy of this object. Keep in mind that
the standard library's :func:`copy` function is a no-op for this class
like for any other python immutable type (eg: :class:`tuple`).
"""
return dict(self)
def __copy__(self):
return self
What distinguishes this from regular inheritance/subclassing and how exactly is the multiple inheritance problem avoided using this approach?
