Bad unsubscriptable-object error

[nickdrozd]

Steps to reproduce

def subscriptable(flag):
    state = None
    for _ in [0, 1]:
        if state is None:
            if flag:
                state = [None]
        elif state[0] is None:
            print('subscripted')

Current behavior

E: 7,13: Value 'state' is unsubscriptable (unsubscriptable-object)

Expected behavior

state is subscriptable by the time it gets subscripted, so that
error is wrong. After all, the code runs:

>>> subscriptable(False)
>>> subscriptable(True)
subscripted

That said, it does look like it could be an error, or maybe it would
be an error in a more complicated case, so maybe a warning could be
emitted -- possibly-unsubscriptable-object or something like that.

Related to, but more specific than, #701

pylint --version output

pylint 2.0.0
astroid 2.0.0
Python 3.6.5 (default, Mar 30 2018, 06:42:10) 
[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.39.2)]

[PCManticore]

From the point of view of pylint, state can also be None as it doesn't consider control flow when inferring the potential values.

[nickdrozd]

I've come around to the belief that control flow should be as statically analyzable as possible. The example code here is terrible in that regard, even if it does happen to work out at runtime. So the warning is appropriate.