On Jul 25, 2010, at 11:15 AM, Alex Gaynor wrote:
Recently I've been wondering why __contains__ casts all of it's returns to be boolean values. Specifically I'd like to propose that __contains__'s return values be passed directly back as the result of the `in` operation.
x = y in z # where x is a non boolean.
One of the beautiful aspects of __contains__ is that its simply signature allows it to be used polymorphically throughout the whole language. It would be ashamed to throw-away this virtue so that you can have a operator version of something that should really be a method (like find() for example).
-1 on the proposal because it makes the language harder to grok while conferring only a dubious benefit (replacing well named methods with a non-descriptive use of an operator).
There is no "natural" interpretation of an in-operator returning a non-boolean. If the above snippet assigns "foo" to x, what does that mean? If it assigns -10, what does that mean? Language design is about associating meanings (semantics) with syntax. ISTM, this would be poor design.