Better types for modules

[ilevkivskyi]

In situations with dynamically/conditionally imported modules:

if random() > 0.5:
    import mod1
else:
    import mod2 as mod1

mypy can infer only types.ModuleType for the mod1 node. Another (quite widespread) example is something like:

try:
    import fastmath
except ImportError:
    import math as fastmath

In such case all symbol table information for math and fastmath is "lost". We could support (at least internally) some special type that will hold symbol table information and will be allowed to appear in unions (or support structural joins). For example, in the above case type of fastmath would be Union[Module['/path/to/fastmath'], Module['math']]. Then mypy will be able to infer fastmath.sin as Callable[[float], float].

[gvanrossum]

IIRC you get an error "Name 'mod1' already defined" on the second. The solution is to put a `# type: ignore` there, then mypy will assume the type of the first import. Honestly that seems reasonable to me. But if you really want the Module[] notation, the argument should probably the full module name, not the filename, i.e. Union[Module['fastmath'], Module['math']].

[ilevkivskyi]

then mypy will assume the type of the first import

Actually mypy assumes the type of the (textually) last encountered import

Honestly that seems reasonable to me.

Except that there is a dozen issues on the tracker about try ... except ImportError and similar.

But if you really want the Module[] notation

I am more interested in having a better internal handling of modules first. Having something conceptually equivalent to "a union of symbol tables" would solve many of the existing issues. Then, depending on experience we could consider adding syntactic support for such types.

[ilevkivskyi]

Related #1153 and #1297 (plus another issues mentioned in the second).

[gvanrossum]

OK, you're right.