[Python-3000] Thoughts on collections.Container and collections.Iterable

[Guido van Rossum]

I would need to think more about this. I'm tempted not to do this, and
let these ABCs denote the *explicit* presence of __contains__ and
__iter__, respectively. Something that's iterable but doesn't
implement __contains__ supports the 'in' operator very inefficiently
(through linear search) which we might not want to encourage.

--Guido

On Feb 8, 2008 5:35 PM, Raymond Hettinger <python at rcn.com> wrote:
> I'm wondering if collections.Iterable should inherit from collections.Container"?
>
> In Python, anything that supports __iter__ automatically supports tests using the "in" operator:
>
>    class A:
>        def __iter__(self):
>            for i in (1,2,3):
>                 yield i
>    >>> 2 in A()
>    True
>
> The two concepts are deeply related.  Anywhere we can write "for x in s: ..." we can also write "if x in s: ...".
>
> The new definition of Iterable would look like this:
>
>     class Iterable(Container):
>         __metaclass__ = ABCMeta
>
>         @abstractmethod
>         def __iter__(self):
>             while False:
>                 yield None
>
>         @classmethod
>         def __subclasshook__(cls, C):
>             if cls is Iterable:
>                 if any("__iter__" in B.__dict__ for B in C.__mro__):
>                     return True
>             return NotImplemented
>
>         def __contains__(self, value):
>             for v in self:
>                 if v == value:
>                     return True
>             return False
>
>
> Raymond
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)