nathanwilliams-ct opened a new pull request, #2153:
URL: https://github.com/apache/buildstream/pull/2153
Buildstream implements a custom `FastEnum`[1].
mypy only supports `enum.Enum` (and it's official variations) when it comes
to doing static type checks on enums [2].
We can't subclass Enum in FastEnum to make mypy happy, because Enum doesn't
allow subclassing [3].
So we end up with a bunch of `str`, `int` etc around the codebase,
instead of the appropriate Enum classes like `OverlapAction` or `_Scope`
which are usually recorded separately in the doc strings.
This means we don't get proper static type checking on our Enums :( .
With stub files [4][5] we can lie to mypy about what type the Enum classes
are[6],
so the type hints around the codebase are now correct be corrected.
Now we can have proper static type checking on the custom Enums, Yay!
So
```
class OverlapAction(FastEnum):
ERROR: str
WARNING: str
IGNORE: str
```
gets stubbed as
```
from enum import Enum
class OverlapAction(Enum):
ERROR: str
WARNING: str
IGNORE: str
```
[1] src/buildstream/types.py#L32
[2] https://mypy.readthedocs.io/en/stable/literal_types.html#enums
[3] https://docs.python.org/3/howto/enum.html#restricted-enum-subclassing
[4] https://typing.python.org/en/latest/guides/writing_stubs.html
[5] https://mypy.readthedocs.io/en/stable/stubgen.html
[6] https://github.com/python/mypy/issues/3217
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