[Python-ideas] add __contains__ into the "type" object

[אלעזר]

This suggestion is really problematic IMHO.

"isinstance" is a nominal check. I can't ask "isinstance(x, Callable[int,
int])" because that would imply solving the halting problem. so
"isinstance(x, Y)" does not mean "is it true that x is an element of the
type Y" but rather "is it true that x was created by a constructor of some
superclass of Y". It is not a type-theoretic question but a question of
origin and intent.

With regard to readability, this will be completely confusing for me. "in"
is a question about inclusion in a collection, not some set-theoretic
inclusion. Otherwise we should also as "x in f" as an equivalent to "not
not f(x)", as is in set theory.

Elazar
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