[Python-Dev] dict containment annoyance

[Scott Dial]

Jean-Paul Calderone wrote:
> def blacklisted(o):
>     try:
>         # Is the object contained in the blacklist set?
>         return o in _blacklistset
>     except TypeError:
>         # If it *cannot* be contained in the blacklist set,
>         # then it probably isn't.
>         return False

I feel confident that the parent could've wrote this snippet himself. 
His point was to discuss the semantic logic of "x in y" throwing and 
exception when x can't be in y (because of the TypeError you are 
catching in your snippet).

FWIW, I think the logic of swallowing the TypeError is completely 
reasonable. It is surprising in a way that you are presented with an 
exception. However, I can't say I have ever ran into this.

For my money, it would make the most sense to have your own blacklistset 
type.

class BlacklistSet(dict):
     def __contains__(self, x):
         try:
             hash(x)
         except TypeError:
             return False
         return super(BlacklistSet, self).__contains__(x)

It's however unfortunate that you cannot use the handy curly-braces anymore.

-- 
Scott Dial
scott at scottdial.com
scodial at indiana.edu