Reword the error message related to void functions

[atugushev]

Fixes #3226.

Aims to provide better assistance to users who may be confused when their void functions technically return None.

[ikonst]

Suggested change

	message = "{} does not return a value (or returns None)".format(
	"Function" if name is None else capitalize(name)
	)
	name = capitalize(name or "Function")
	message = f"{name} does not return a value (or returns None)"

[atugushev]

Suggested change

	message = "{} does not return a value (or returns None)".format(
	"Function" if name is None else capitalize(name)
	)
	name = "Function" if name is None else capitalize(name)
	message = f"{name} does not return a value (or returns None)"

Thanks for the suggestion! By using an f-string along with the name variable the code definitely looks better. One note, perhaps we could keep an explicit check for None as in the original code?

[ikonst]

Of course. :)

[ikonst]

(I "simplified" it because an empty string wouldn't make for a good interpolation either, but it also suggests it's a possibility where currently it's not.)

[atugushev]

Considering the implementation of callable_name() it seems to me that we could omit the capitalization:

mypy/mypy/messages.py

Lines 2963 to 2967 in b49be10

	def callable_name(type: FunctionLike) -> str | None:
	name = type.get_name()
	if name is not None and name[0] != "<":
	return f'"{name}"'.replace(" of ", '" of "')
	return name

It always returns either None, "Foo", or <Foo>, where capitalization does not seem to be applicable. What do you think?

[atugushev]

Addressed in dceaac1

[ikonst]

Looks much better!

[hauntsaninja]

Thanks for this PR! I'd vote for a slightly more specific phrase, something like Error: "f" does not return a value (it only ever returns None)

I think this is less likely to confuse users who either don't understand return semantics or from the message think mypy doesn't understand return semantics.

[github-actions]

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

steam.py (https://github.com/Gobot1234/steam.py)
- steam/protobufs/__init__.py:77: error: "setattr" does not return a value  [func-returns-value]
+ steam/protobufs/__init__.py:77: error: "setattr" does not return a value (it only ever returns None)  [func-returns-value]
- steam/protobufs/__init__.py:78: error: "setattr" does not return a value  [func-returns-value]
+ steam/protobufs/__init__.py:78: error: "setattr" does not return a value (it only ever returns None)  [func-returns-value]
- steam/gateway.py:165: error: "info" of "Logger" does not return a value  [func-returns-value]
+ steam/gateway.py:165: error: "info" of "Logger" does not return a value (it only ever returns None)  [func-returns-value]
- steam/gateway.py:185: error: "info" of "Logger" does not return a value  [func-returns-value]
+ steam/gateway.py:185: error: "info" of "Logger" does not return a value (it only ever returns None)  [func-returns-value]
- steam/ext/csgo/protobufs/__init__.py:18: error: "setattr" does not return a value  [func-returns-value]
+ steam/ext/csgo/protobufs/__init__.py:18: error: "setattr" does not return a value (it only ever returns None)  [func-returns-value]
- steam/ext/tf2/protobufs/__init__.py:17: error: "setattr" does not return a value  [func-returns-value]
+ steam/ext/tf2/protobufs/__init__.py:17: error: "setattr" does not return a value (it only ever returns None)  [func-returns-value]

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/filesystems.py:494: error: Function does not return a value  [func-returns-value]
+ src/prefect/filesystems.py:494: error: Function does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/filesystems.py:587: error: Function does not return a value  [func-returns-value]
+ src/prefect/filesystems.py:587: error: Function does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/filesystems.py:723: error: Function does not return a value  [func-returns-value]
+ src/prefect/filesystems.py:723: error: Function does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/filesystems.py:821: error: Function does not return a value  [func-returns-value]
+ src/prefect/filesystems.py:821: error: Function does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/blocks/notifications.py:393: error: "append" of "list" does not return a value  [func-returns-value]
+ src/prefect/blocks/notifications.py:393: error: "append" of "list" does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/blocks/notifications.py:395: error: "append" of "list" does not return a value  [func-returns-value]
+ src/prefect/blocks/notifications.py:395: error: "append" of "list" does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/blocks/notifications.py:397: error: "append" of "list" does not return a value  [func-returns-value]
+ src/prefect/blocks/notifications.py:397: error: "append" of "list" does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/blocks/notifications.py:399: error: "append" of "list" does not return a value  [func-returns-value]
+ src/prefect/blocks/notifications.py:399: error: "append" of "list" does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/engine.py:1451: error: Function does not return a value  [func-returns-value]
+ src/prefect/engine.py:1451: error: Function does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/server/models/flow_run_notification_policies.py:47: error: "flush" of "Session" does not return a value  [func-returns-value]
+ src/prefect/server/models/flow_run_notification_policies.py:47: error: "flush" of "Session" does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/server/models/configuration.py:29: error: "flush" of "Session" does not return a value  [func-returns-value]
+ src/prefect/server/models/configuration.py:29: error: "flush" of "Session" does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/server/models/concurrency_limits_v2.py:94: error: "flush" of "Session" does not return a value  [func-returns-value]
+ src/prefect/server/models/concurrency_limits_v2.py:94: error: "flush" of "Session" does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/server/models/concurrency_limits_v2.py:217: error: "flush" of "Session" does not return a value  [func-returns-value]
+ src/prefect/server/models/concurrency_limits_v2.py:217: error: "flush" of "Session" does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/server/models/agents.py:39: error: "flush" of "Session" does not return a value  [func-returns-value]
+ src/prefect/server/models/agents.py:39: error: "flush" of "Session" does not return a value (it only ever returns None)  [func-returns-value]
- src/prefect/server/services/scheduler.py:210: error: "rollback" of "Session" does not return a value  [func-returns-value]
+ src/prefect/server/services/scheduler.py:210: error: "rollback" of "Session" does not return a value (it only ever returns None)  [func-returns-value]

streamlit (https://github.com/streamlit/streamlit)
- lib/tests/streamlit/elements/slider_test.py:200:9: error: "assertEqual" of "TestCase" does not return a value  [func-returns-value]
+ lib/tests/streamlit/elements/slider_test.py:200:9: error: "assertEqual" of "TestCase" does not return a value (it only ever returns None)  [func-returns-value]

discord.py (https://github.com/Rapptz/discord.py)
- discord/channel.py:660: error: Function does not return a value  [func-returns-value]
+ discord/channel.py:660: error: Function does not return a value (it only ever returns None)  [func-returns-value]

ibis (https://github.com/ibis-project/ibis)
- ibis/examples/gen_registry.py:75: error: "make_descriptions" does not return a value  [func-returns-value]
+ ibis/examples/gen_registry.py:75: error: "make_descriptions" does not return a value (it only ever returns None)  [func-returns-value]

[hauntsaninja]

I committed the wording tweak, thanks again for this PR! :-)

[atugushev]

Thanks @hauntsaninja!