Typehint importing module dynamically using importlib

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Give something as follows:

import importlib

module_path = "mod"
mod = importlib.import_module(module_path, package=None)
print(mod.Foo.Bar.x)

where mod.py is:

class Foo:
    class Bar:
        x = 1

mypy file.py --strict raises the following error:

file.py:7: error: Module has no attribute "Foo"  [attr-defined]

I'm wondering how one is supposed to go about type-hinting this, or if this is something which would typically just be ignored with # type: ignore[attr-defined] (assuming that the code is necessary, and the only options are type-hinting or ignoring the type-hint)?

Why I am using importlib in this situation

The way that importlib is being used is that there's some path:

x.y.<changes>.z

Where <changes> is dynamic, but the others are fixed. I'm confident that the module will contain the attributes which are being called, but due to <changes>, importlib is used for the import.

Which might be summarised as:

I do not know precisely which module I will be importing, but I know it will have a class Foo in it.

python-typing python-importlib mypy type-hinting python
1 Answers

As was alluded to by @MisterMiyagi in the comments, I think the solution here is to use structural, rather than nominal, subtyping. Nominal subtyping is where we use direct class inheritance to define type relationships. For example, collections.Counter is a subtype of dict because it directly inherits from dict. Structural subtyping, however, is where we define types based on certain properties a class has or certain behaviours it displays. int is a subtype of typing.SupportsFloat not because it directly inherits from SupportsFloat (it doesn't), but because SupportsFloat is defined as a certain interface, and int satisfies that interface.

When type-hinting, we can define structural types using typing.Protocol. You could satisfy MyPy in this situation like this:

import importlib
from typing import cast, Protocol

class BarProto(Protocol):
    x: int
    
class FooProto(Protocol):
    Bar: type[BarProto]
    
class ModProto(Protocol):
    Foo: type[FooProto]

module_path = "mod"
mod = cast(ModProto, importlib.import_module(module_path, package=None))

print(mod.Foo.Bar.x)

reveal_type(mod)
reveal_type(mod.Foo)
reveal_type(mod.Foo.Bar)
reveal_type(mod.Foo.Bar.x)

We've defined several interfaces here:

  • BarProto: in order to satisfy this interface, a type has to have an attribute x that's of type int.
  • FooProto: in order to satisfy this interface, a type has to have an attribute Bar that is a class of which instances satisfy the BarProto protocol.
  • ModProto: in order to satisfy this interface, a type has to have an attribute Foo that is a class of which instances satisfy the FooProto protocol.

Then, when importing the module, we use typing.cast to assert to the type-checker that the module we're importing satisfies the ModProto protocol.

Run it through MyPy, and it informs us it has inferred the following types:

main.py:18: note: Revealed type is "__main__.ModProto"  
main.py:19: note: Revealed type is "Type[__main__.FooProto]"
main.py:20: note: Revealed type is "Type[__main__.BarProto]"
main.py:21: note: Revealed type is "builtins.int"

Read more about structural subtyping in python here and here.

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