Implicit re-exports should not be disabled for installed PEP 561 packages

[GPHemsley]

With:

• Python 3.6.10
• mypy 0.770
• requests 2.23.0
• tenacity 6.2.0

the following code:

#!/usr/bin/env python3

from typing import (
	Any,
	Callable,
)

import requests
import tenacity

# Keep trying the request until we connect.
@tenacity.retry(retry=tenacity.retry_if_exception_type(requests.exceptions.ConnectionError))
def retry_if_connection_error(func: Callable[..., requests.Response], *args: Any, **kwargs: Any) -> requests.Response:
	return func(*args, **kwargs)

# Keep trying the request unless we connect.
@tenacity.retry(retry=tenacity.retry_unless_exception_type(requests.exceptions.ConnectionError))
def retry_unless_connection_error(func: Callable[..., requests.Response], *args: Any, **kwargs: Any) -> requests.Response:
	return func(*args, **kwargs)

produces the following output:

$ mypy --strict scripts/dev_test_mypy_tenacity.py
scripts/dev_test_mypy_tenacity.py:12: error: Module has no attribute "retry_if_exception_type"; maybe "retry_if_exception", "retry_unless_exception_type", or "retry_if_exception_message"?
    @tenacity.retry(retry=tenacity.retry_if_exception_type(requests.exceptions.ConnectionError))
                          ^
scripts/dev_test_mypy_tenacity.py:17: error: Module has no attribute "retry_unless_exception_type"; maybe "retry_if_exception_type"?
    @tenacity.retry(retry=tenacity.retry_unless_exception_type(requests.exceptions.ConnectionError))
                          ^
Found 2 errors in 1 file (checked 1 source file)

Note that each error message suggests an alternative that the other error message insists does not exist.

Tenacity only just added the bare minimum of type hints (jd/tenacity#221), so I don't know if there's some conflict there, but even if that's the case, this seems like a bug in mypy.

Possibly related mypy issues: #8220, #8210, #7125, #7029, #6551, #4930

[JukkaL]

This seems to be caused by --no-implicit-reexport. Implicit re-exports should always be allowed in PEP 561 packages, but that's not the case right now.

As a workaround, you can run mypy with --strict --implicit-reexport.

[hauntsaninja]

Note that #9237 should improve the error message here

[gaborbernat]

Implicit re-exports should always be allowed in PEP 561 packages, but that's not the case right now.

How come @JukkaL? I think implicit re-export should be an opt-in, not always-on feature. I could imagine py.typed file containing some flag to enable/disable this.

[erictraut]

Implicit re-exports should always be allowed in PEP 561 packages, but that's not the case right now.

This contradicts what (I thought) we had agreed to in the typing-sig discussion on this topic. If a package claims to be py.typed, then it needs to follow typing rules and be explicit about what symbols are exported from each submodule. Pyright assumes that py.typed packages never use implicit re-exports. Here is the guidance we have been providing to package authors, and pyright's implementation is consistent with this guidance. I would encourage mypy to default to the same behavior to provide consistency for package authors.

[hauntsaninja]

Between #9237 / #13925 / #13967 / #16129 I think we've done everything I'd want to do here