Error when using write hooks in python < 3.9

[CaselIT]

Hi, I have an issue which boils down to the same point in alembic code, but I'm not using any development-only script, but [post_write_hooks] feature.

Relevant part of alembic.ini:

[post_write_hooks]

# format using "black" - use the console_scripts runner, against the "black" entrypoint
hooks = black, isort
black.type = console_scripts
black.entrypoint = black
black.options = REVISION_SCRIPT_FILENAME

isort.type = console_scripts
isort.entrypoint = isort
isort.options = REVISION_SCRIPT_FILENAME

16:18 $ poetry run alembic revision --autogenerate -m "Add foobar"
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
  Generating /home/foobar/myproject/api/migrations/versions/2021_09_27_6c36df7b6df9_add_foobar.py ...  done
  Running post write hook "black" ...
  FAILED
Traceback (most recent call last):
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/bin/alembic", line 8, in <module>
    sys.exit(main())
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/config.py", line 588, in main
    CommandLine(prog=prog).main(argv=argv)
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/config.py", line 582, in main
    self.run_cmd(cfg, options)
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/config.py", line 559, in run_cmd
    fn(
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/command.py", line 234, in revision
    scripts = [script for script in revision_context.generate_scripts()]
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/command.py", line 234, in <listcomp>
    scripts = [script for script in revision_context.generate_scripts()]
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/autogenerate/api.py", line 600, in generate_scripts
    yield self._to_script(generated_revision)
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/autogenerate/api.py", line 505, in _to_script
    return self.script_directory.generate_revision(
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/script/base.py", line 736, in generate_revision
    write_hooks._run_hooks(path, post_write_hooks)
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/script/write_hooks.py", line 84, in _run_hooks
    util.status(
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/util/messaging.py", line 56, in status
    ret = fn(*arg, **kw)
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/script/write_hooks.py", line 58, in _invoke
    return hook(revision, options)
  File "/home/foobar/.cache/pypoetry/virtualenvs/someenv-py3.8/lib/python3.8/site-packages/alembic/script/write_hooks.py", line 145, in console_scripts
    "import %s; %s.%s()" % (impl.module, impl.module, impl.attr),
AttributeError: 'EntryPoint' object has no attribute 'module'

The origin of the issue is of course the same, alembic/script/write_hooks.py seems to be working with python 3.9+ as discovered above:

This happens, because the code assumes (write_hooks.py:141), that the returned object EntryPoint has a module or attr property (which it has on 3.9, but not on 3.8).

While write_pyi.py is an script for development use only, I guess that all post-write hooks deal with the same issue as well?

Originally posted by @antoni-szych-rtbhouse in #915 (comment)

[CaselIT]

The issue can be solved by installing importlib_metadata also on 3.8.

I wonder why the test did not catch it. I guess we need to add a covering case for this

[jmrodriguez-lab]

I'm on Python 3.8.8 with alembic==1.7.3 and installing importlib_metadata does not solve the issue.
Maybe there's something else to install? (this error happens with the black hook from the .ini example).

Any hint would be appreciated :-)

[CaselIT]

Sorry, I was not clear, a change in the code is still needed, changing this condition to if py39

alembic/alembic/util/compat.py

Lines 32 to 35 in d95e15c

	if py38:
	from importlib import metadata as importlib_metadata
	else:
	import importlib_metadata # type:ignore[no-redef] # noqa

will try to solve today so we can hopefully release a fix

[sqla-tester]

Federico Caselli has proposed a fix for this issue in the master branch:

Fix write hook issue in python < 3.9. https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/3115