__contains__ vs has_key() - am I missing a subtle distinction?

[Alex Martelli]

Skip Montanaro wrote:

> I got to wondering if there is a difference between __contains__() and
> has_key() for dictionaries (I don't think there is), so I peeked at
> UserDict.UserDict and saw they were implemented as distinct methods:
> 
>     def has_key(self, key):
>         return self.data.has_key(key)
> 
>     def __contains__(self, key):
>         return key in self.data
> 
> even though the UserDict implementation explicitly defines self.data as a
> true dict.  There can be no mistake what the semantics of the two methods
> are.  I half expected __contains__ to be defined as
> 
>     __contains__ = has_key
> 
> Did I miss some subtle distinction or was the UserDict author just trying
> to be as explicit as possible in demonstrating the relationship between
> methods and language constructs?

I suspect the latter.  There are no differences now, and I don't think
any were ever planned from back when __contains__ was introduced.  But
asking on python-dev may give you a better chance to hear from the
original author.


Alex