Common Python Idioms

[Marc 'BlackJack' Rintsch]

In <elbkp8$ket$1 at news.sap-ag.de>, Daniel Dittmar wrote:

> John Machin wrote:
>> No it doesn't look wrong to anyone who has read the docs on
>> sys.modules. 
> 
> My point was really that there is no obvious implementation for 'in' on 
> dictionaries, so it should have been left out. And that GvR thought so 
> for quite some time as well (until he got mixed up with a crowd of ex 
> C++ programmers).

Maybe there's no obvious implementation for ``in`` and dictionaries but
there is the semantics of ``in`` and iterables.  Dictionaries are iterable
so it's clearly defined what ``'answer' in {'answer': 42}`` should return.

In [84]: class A:
   ....:     def __iter__(self):
   ....:         return iter((1,2,3))
   ....:

In [85]: a = A()

In [86]: 2 in a
Out[86]: True

In [87]: 5 in a
Out[87]: False

If ``in`` shouldn't work with dictionaries, either `__contains__()` must
be implemented to throw an exception or dictionaries shouldn't be iterable.

Ciao,
	Marc 'BlackJack' Rintsch