mypy failure with --no-implicit-reexport

[philpep]

Describe the bug

Using mypy with --no-implicit-reexport (which is part of --strict) generate a failure since alembic 1.7

Expected behavior

No AssertionError

To Reproduce

$ cat << EOF > t.py
import alembic
EOF
$ mypy --no-implicit-reexport t.py
Traceback (most recent call last):
  File "[...]/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "[...]/lib/python3.9/site-packages/mypy/__main__.py", line 11, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "mypy/main.py", line 87, in main
  File "mypy/main.py", line 165, in run_build
  File "mypy/build.py", line 179, in build
  File "mypy/build.py", line 254, in _build
  File "mypy/build.py", line 2697, in dispatch
  File "mypy/build.py", line 3021, in process_graph
  File "mypy/build.py", line 3138, in process_stale_scc
  File "mypy/build.py", line 2288, in write_cache
  File "mypy/build.py", line 1475, in write_cache
  File "mypy/nodes.py", line 313, in serialize
  File "mypy/nodes.py", line 3149, in serialize
  File "mypy/nodes.py", line 3083, in serialize
AssertionError: Definition of alembic.runtime.migration.EnvironmentContext is unexpectedly incomplete

Versions.

• OS: Debian 11
• Python: 3.9
• Alembic: 1.7.1
• SQLAlchemy: 1.4.23
• mypy: 0.910

Note it might be a mypy bug (since this is an AssertionError), but I think this looks related to some weirdness in alembic typing code.

Thanks!

[CaselIT]

Hi,

Thanks for reporting. I'm not sure what we should change.
I'm not sure if empty re-export files https://github.com/sqlalchemy/alembic/blob/master/alembic/environment.py and https://github.com/sqlalchemy/alembic/blob/master/alembic/migration.py are to blame.

Running it without --no-implicit-reexport works for you?

[philpep]

Yes, running mypy without --no-implicit-reexport or --strict doesn't trigger the bug.

Seems when TYPE_CHECKING is enabled there's an import loop between alembic.runtime.migration and alembic.runtime.environment but I'm not sure this is related to the bug since commenting the lines still trigger the bug.
I also tried to comment re-export imports from alembic.environment and alembic.migration and this does not have much effect.

[philpep]

A workaround for mypy config might be:

[mypy-alembic.*]
no_implicit_reexport = false

[zzzeek]

IMO mypy devs should look at this. maybe @bryanforbes has insight

[taybin]

I'm not sure if this is relevant to this particular issue, but with no_implicit_reexport set to false, mypy still complains that:

alembic/env.py:9: error: Module has no attribute "config"; maybe "configure"?

[CaselIT]

I'm not sure if this is relevant to this particular issue, but with no_implicit_reexport set to false, mypy still complains that:

alembic/env.py:9: error: Module has no attribute "config"; maybe "configure"?

this is explicetly skipped by the stub generator here https://github.com/sqlalchemy/alembic/blob/master/tools/write_pyi.py#L22 since it has no documentation. I guess I can try fiddle with it to see if that's the problem.

[CaselIT]

the issue seems to be in caused by the presence of context.pyi, and in particular it seems a circular import error.

To reproduce the error it's sufficient having as context.pyi:

# context.pyi
from .migration import MigrationContext

I think this is a mypy bug