Marked the std streams as possibly be None

[srittau]

Closes: #11778

[srittau]

Looking at the primer output:

With the exception of cwltool (and streamlit, where only the error message has changed), these are all unintended, but positive side-effects of this change. In all cases, valid objects that were not derived from TextIO were assigned to the streams. We should probably look into replacing TextIO with a protocol at some point.

The cwltool tests assign a string to the streams. This is now unfortunately a false negative, but still worth it, in my opinion.

[mondeja]

Note that __stdin__, __stdout__ and __stderr__ may also be None. See the linked issue.

[srittau]

Good catch. I will try using regular None for now. I think this makes sense for the underscored attributes.

[github-actions]

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

rich (https://github.com/Textualize/rich)
+ rich/console.py:93: error: Item "None" of "TextIOWrapper | None" has no attribute "fileno"  [union-attr]
+ rich/console.py:97: error: Item "None" of "TextIOWrapper | None" has no attribute "fileno"  [union-attr]
+ rich/console.py:101: error: Item "None" of "TextIOWrapper | None" has no attribute "fileno"  [union-attr]
- rich/live.py:180: error: Subclass of "TextIO" and "FileProxy" cannot exist: would have incompatible method signatures  [unreachable]
- rich/live.py:183: error: Subclass of "TextIO" and "FileProxy" cannot exist: would have incompatible method signatures  [unreachable]

pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/capture.py:204: error: Item "None" of "TextIOWrapper | None" has no attribute "encoding"  [union-attr]
+ src/_pytest/faulthandler.py:65: error: Item "None" of "TextIOWrapper | None" has no attribute "fileno"  [union-attr]

tornado (https://github.com/tornadoweb/tornado)
+ tornado/test/runtests.py:174: error: Unused "type: ignore" comment  [unused-ignore]

werkzeug (https://github.com/pallets/werkzeug)
- tests/test_debug.py:196: error: Incompatible types in assignment (expression has type "HTMLStringO", variable has type "TextIO")  [assignment]
- tests/test_debug.py:199: error: "TextIO" has no attribute "reset"  [attr-defined]
- tests/test_debug.py:201: error: "TextIO" has no attribute "reset"  [attr-defined]
- tests/test_debug.py:213: error: Incompatible types in assignment (expression has type "HTMLStringO", variable has type "TextIO")  [assignment]
- tests/test_debug.py:216: error: "TextIO" has no attribute "reset"  [attr-defined]

scrapy (https://github.com/scrapy/scrapy)
+ scrapy/utils/log.py:127: error: Unused "type: ignore" comment  [unused-ignore]

cwltool (https://github.com/common-workflow-language/cwltool)
+ tests/test_context.py:17: error: Unused "type: ignore" comment  [unused-ignore]
+ tests/test_context.py:18: error: Unused "type: ignore" comment  [unused-ignore]

streamlit (https://github.com/streamlit/streamlit)
- lib/tests/streamlit/web/bootstrap_test.py:107:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:107:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:131:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:131:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:153:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:153:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:178:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:178:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:203:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:203:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:224:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:224:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:248:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:248:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:273:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:273:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:298:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:298:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:315:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:315:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:336:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:336:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]
- lib/tests/streamlit/web/bootstrap_test.py:345:15: error: "TextIO" has no attribute "getvalue"  [attr-defined]
+ lib/tests/streamlit/web/bootstrap_test.py:345:15: error: Item "TextIO" of "Union[TextIO, Any]" has no attribute "getvalue"  [union-attr]

[bwoodsend]

Is there are reason why these are MaybeNone instead of None? I expect running mypy on sys.stdout.write("...") without an appropriate if sys.stdout is not None guard to tell me of the NoneType has no attribute write error that I would get from running that snippet of code using pythonw.exe. But instead it tells me that everything is fine.