How to handle convenience imports with Mypy

Python convenience imports are where objects are imported into a package’s __init__.py module so client code has a simple, easy-to-remember location to import objects from.

But, by default, Mypy will complain about such imports when run in strict mode.

Example

For example, consider a foo.__init__ module with contents:

# foo/__init__.py
from ._queries import my_function

and a main.py module that imports my_function from the foo package:

# main.py
from foo import my_function
...

Running mypy --strict on main.py yields an error:

$ mypy --strict main.py
main.py:1: error: Module "foo" does not explicitly export attribute "my_function"  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

This error relates to the no_implicit_reexport setting which is enabled by Mypy’s strict mode. This instructs Mypy to not consider objects are exported unless:

• the item is imported using from ... import ... as;
• the item is included in the __all__ module variable.

The above foo.__init__ module does not meet either of these criteria. If we want to conform to strict mode, the best approach is probably to add the objects we want to reexport to a __all__ variable:

# foo/__init__.py
from ._queries import my_function

__all__ = ["my_function"]

Patching module objects in tests

This is also a problem when using mock.patch to patch module objects in a test.

For example, Mypy (in strict mode) will complain that this test attempts to replace the requests object with a stub:

# tests/test_vendors.py
from unittest import mock
from foo import vendors

@mock.patch.object(vendors, "requests")
def test_vendor_client(requests):
    # Stub some responses etc
    requests.post.return_value = ...

where foo/vendors.py imports the requests package:

# foo/vendors.py
import requests
...

This generates a attr-defined error:

$ mypy tests/test_vendors.py
tests/test_vendors.py:4: error: "Module foo.vendors" does not explicitly export attribute "requests"  [attr-defined]

As before, importing requests like this indicates to Mypy that it isn’t available for re-export and so other modules, like our test module, cannot reference it.

Since this is a common pattern in tests, it’s better to ignore the error rather than change the way packages are imported (e.g. to import requests as requests).

This can be done in your Mypy config file:

# setup.cfg
[mypy]
strict = true

[mypy-tests.*]
disable_error_code = attr-defined

This requires Mypy v0.991 or above to work.