mypy class forward references in type alias gives error when in other module

user_253954 0
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I want to keep my type aliases in one module, say my_types, to be able to use them anywhere in my application (similar to the standard typing module). But mypy complains that the forward reference to class X is not defined. If I define class X later in that same module, it’s okay, but if it defined in another one, mypy gets upset.

So my question is, how do I keep all my type aliases in one module without mypy producing an error about forward references that are not defined in the same module? Or is that a wrong approach somehow?

Here is my example code:

from my_types import SomeXs

class X:
    pass

Type aliases are defined like so:

# my_types.py
from typing import List

SomeXs = List['X']

When I run mypy, I get an error that X is not defined:

$ mypy module.py
my_types.py:4: error: Name 'X' is not defined
Found 1 error in 1 file (checked 1 source file)
mypy python
2 Answers

I’m gonna share the solution I found in the mypy documentation common issues section here: It is necessary for mypy to have access to the definition of X. To avoid an import cycle, the mypy documentation recommends a trick - only import the definition that would create a cyclic import when type checking. It goes like this:

from typing import List, TYPE_CHECKING

if TYPE_CHECKING:
    from main import X

SomeXs = List['X']

This makes the Python interpreter ignore the import when executing this code, but mypy still uses the definition of X from main.

Et voilá:

$ mypy *.py
Success: no issues found in 2 source files
user_2539540

Mypy needs to have referenced types in scope:

$ cat my_types.py 
from typing import List

from main import X

SomeXs = List[X]

And the code containing X doesn't reference SomeXs, so it shouldn't be imported there:

$ cat main.py 
class X:
    pass

The result:

$ mypy .
Success: no issues found in 2 source files
user_965880
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