Makes Enum members implicitly final, refs #5599

[sobolevn]

Makes Enum members implicitly final

My problem was that this code was typechecking:

from enum import Enum

class A(str, Enum):
    some = 'some'
    other = 'other'

A.some = 'new'

While in runtime it was failing with:

Traceback (most recent call last):
  File "ex.py", line 12, in <module>
    A.some = 'new'
  File "/Users/sobolev/.pyenv/versions/3.8.9/lib/python3.8/enum.py", line 427, in __setattr__
    raise AttributeError('Cannot reassign members.')
AttributeError: Cannot reassign members.

This change allows to catch this error by making all Enum members implicitly Final.

I also had to modify Enum plugin, since it was not ready to work with Literal[True] and Literal[False].

Closes #5599

Notes

I would love to hear your feedback! 👍

[sobolevn]

alerta (https://github.com/alerta/alerta.git)
alerta/models/enums.py:159: error: Cannot override writable attribute "value" with a final one

This one is interesting:

class A(str, Enum):
    value = 'value'

Produces:

error: Cannot override writable attribute "value" with a final one

[sobolevn]

Is this fail related?

__________________________ testTypedPkgSimple_python2 __________________________

[gw1] linux -- Python 3.7.1 /home/travis/build/python/mypy/.tox/py/bin/python

data: /home/travis/build/python/mypy/test-data/unit/pep561.test:85:

/home/travis/build/python/mypy/mypy/test/testpep561.py:43: in run_case

    test_pep561(test_case)

/home/travis/build/python/mypy/mypy/test/testpep561.py:116: in test_pep561

    with virtualenv(python) as venv:

/opt/python/3.7.1/lib/python3.7/contextlib.py:112: in __enter__

    return next(self.gen)

/home/travis/build/python/mypy/mypy/test/testpep561.py:63: in virtualenv

    raise Exception("Failed to create venv. Do you have virtualenv installed?\n" + err)

E   Exception: Failed to create venv. Do you have virtualenv installed?

E   Already using interpreter /usr/bin/python2

E   New python executable in /tmp/tmppdnp09_w/bin/python2

E   Also creating executable in /tmp/tmppdnp09_w/bin/python

E   Command /tmp/tmppdnp09_w/bin/python2 -m pip config list had error code 1

E   Installing setuptools, pip, wheel...

E   

E     Complete output from command /tmp/tmppdnp09_w/bin/python2 - setuptools pip wheel:

E     Traceback (most recent call last):

E     File "<stdin>", line 15, in <module>

E     File "/home/travis/build/python/mypy/.tox/py/lib/python3.7/site-packages/virtualenv_support/pip-21.1.3-py3-none-any.whl/pip/__init__.py", line 1, in <module>

E   ImportError: No module named typing

E   ----------------------------------------

E   ...Installing setuptools, pip, wheel...done.

E   Running virtualenv with interpreter /opt/pyenv/shims/python2

E   Traceback (most recent call last):

E     File "/home/travis/build/python/mypy/.tox/py/lib/python3.7/site-packages/virtualenv.py", line 2644, in <module>

E       main()

E     File "/home/travis/build/python/mypy/.tox/py/lib/python3.7/site-packages/virtualenv.py", line 869, in main

E       symlink=options.symlink,

E     File "/home/travis/build/python/mypy/.tox/py/lib/python3.7/site-packages/virtualenv.py", line 1188, in create_environment

E       install_wheel(to_install, py_executable, search_dirs, download=download)

E     File "/home/travis/build/python/mypy/.tox/py/lib/python3.7/site-packages/virtualenv.py", line 1028, in install_wheel

E       _install_wheel_with_search_dir(download, project_names, py_executable, search_dirs)

E     File "/home/travis/build/python/mypy/.tox/py/lib/python3.7/site-packages/virtualenv.py", line 1125, in _install_wheel_with_search_dir

E       call_subprocess(cmd, show_stdout=False, extra_env=env, stdin=script)

E     File "/home/travis/build/python/mypy/.tox/py/lib/python3.7/site-packages/virtualenv.py", line 962, in call_subprocess

E       raise OSError("Command {} failed with error code {}".format(cmd_desc, proc.returncode))

E   OSError: Command /tmp/tmppdnp09_w/bin/python2 - setuptools pip wheel failed with error code 1

It does not seem to be.

[hauntsaninja]

It's unrelated, @pranavrajpal fixed this in #10853

[sobolevn]

Rebased, now it should be fine.

[bluetech]

I was just intending to implement this myself, but luckily I checked before and saw there's already a PR for it :)

My reason for wanting this is not so much marking the values immutable, but to have A.some.value typed as Literal['some'] instead of str, which is something I found to be useful several times already. Currently it works by doing

class A(Enum):
    some: Final = 'some'
    other = 'other'

reveal_type(A.some.value)  # Literal['some']
reveal_type(A.other.value)  # str

but no reason for it to not be implicit.

[JukkaL]

Nice, enum members are clearly not mutable. This looks good to me overall, but I have question about one change.

[JukkaL]

It's unclear to me when the above new code is needed. Is there a test case that depends on this? If not, could you add a test case.

In any case, it would be good to have a comment here.

[sobolevn]

Here are related changes:

1. https://github.com/python/mypy/pull/10852/files#diff-ebc735ad8fa6eb42d5390fa4fa9ef60581764c7bbfb5eafcde16f5e40c330c84R69
2. https://github.com/python/mypy/pull/10852/files#diff-ebc735ad8fa6eb42d5390fa4fa9ef60581764c7bbfb5eafcde16f5e40c330c84R143
3. https://github.com/python/mypy/pull/10852/files#diff-ebc735ad8fa6eb42d5390fa4fa9ef60581764c7bbfb5eafcde16f5e40c330c84R102

[sobolevn]

Updated code with more comments 👍

[hauntsaninja]

Thanks, this is a nice improvement. Lgtm, but wouldn't mind if Jukka took another look.

[sobolevn]

Rebased! 🚜

[sobolevn]

Merge conflicts 😮‍💨

Should be fine now!

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

prefect (https://github.com/PrefectHQ/prefect.git)
+ src/prefect/core/task.py:45: error: Cannot override writable attribute "value" with a final one

alerta (https://github.com/alerta/alerta.git)
+ alerta/models/enums.py:159: error: Cannot override writable attribute "value" with a final one

[97littleleaf11]

lgtm, I really like this feature! Also thanks for your detailed comments.

[sobolevn]

Awesome, thanks, everyone! 🎉

[joey-laminar]

I was just intending to implement this myself, but luckily I checked before and saw there's already a PR for it :)

My reason for wanting this is not so much marking the values immutable, but to have A.some.value typed as Literal['some'] instead of str, which is something I found to be useful several times already. Currently it works by doing

class A(Enum):
    some: Final = 'some'
    other = 'other'

reveal_type(A.some.value)  # Literal['some']
reveal_type(A.other.value)  # str

but no reason for it to not be implicit.

It looks like this still doesn't work in latest mypy for strings (but does work for int/bool). Any idea why?