The behavior of `--no-explicit-reexport` is inconsistent/misleading

[pavel-kirienko]

Bug Report

The behavior of --no-explicit-reexport is inconsistent/misleading, see MWE below.

To Reproduce

Define the following package of four files:

├── a
│   ├── __init__.py
│   └── X.py
├── c.py
└── d.py

a/__init__.py:

from a.X import X as X
Y = X

a/X.py

class X:
    pass

c.py

from a import Y as W

d.py

from c import W

Expected Behavior

$ mypy --strict --no-implicit-reexport .
Success: no issues found in 4 source files

Actual Behavior

$ mypy --strict --no-implicit-reexport .
d.py:1: error: Module "c" does not explicitly export attribute "W"; implicit reexport disabled
Found 1 error in 1 file (checked 4 source files)

The docs state that the form import ... as ... is sufficient to mark the imported item for re-export, so this is expected to work. However, the most suspicious part is that if you modify c.py (sic!) as follows:

-- from a import Y as W
++ from a import X as W

then MyPy reports no error.

One would expect both cases to be accepted.

Note that if a is not a package, both cases yield a re-export error.

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: --strict --no-implicit-reexport
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10
• Operating system and version: GNU/Linux Manjaro 21.2

[hauntsaninja]

Thanks, can repro the inconsistency on master, the expected behaviour is an error in both cases. This was somewhat recently clarified in PEP 484 (https://github.com/python/peps/pull/1630/files)

[hauntsaninja]

I'd welcome a corresponding update to our docs, if you want to make a PR for it! :-)

[pavel-kirienko]

Thanks, can repro the inconsistency on master, the expected behaviour is an error in both cases. This was somewhat recently clarified in PEP 484

This is a bit unexpected. What is the recommended pattern for re-exporting an entity under a different name? The following obvious approach looks verbose:

from x import y
z = y

[hauntsaninja]

Yup, that's what we used in typeshed :-)

[hauntsaninja]

Okay, I made #11707 to fix the inconsistency and it looks like #11083 updated the docs already