baseline accumulate

[KotlinIsland]

like when you wanna run on multiple platforms / multiple directories

> mypy --write-baseline .
a
b
c
found 3 errors (3 new)

> mypy --baseline-add --platform win32 .
w
found 3 errors (1 new)

I suppose this would set a flag in the baseline file indicating that auto-baseline shouldn't be used. Or perhaps additional info could be stored on each error indicating it's platform/version exclusivity.

Workaround

Have different baseline files for each invocation:

> mypy --platform lignux --write-baseline --baseline-file .mypy/baseline-lignux.json .
...

> mypy --platform win32 --write-baseline --baseline-file .mypy/baseline-win32.json .
...