bpo-41322: add two unit tests for deprecation of test return values

[akulakov]

tests for #27748

https://bugs.python.org/issue41322

[serhiy-storchaka]

While we are here, please document this change in the unittest module documentation and add an entry in the What's New document.

[serhiy-storchaka]

I would also add stacklevel=3 in calls of warnings.warn() and change tests to test it:

with self.assertWarns(DeprecationWarning) as w:
    Foo('test_func').run()
self.assertIn('It is deprecated to return a value!=None', w.warnings[0].message)
self.assertIn('test_func', w.warnings[0].message)
self.assertEqual(w.warnings[0].filename, __file__)

[akulakov]

While we are here, please document this change in the unittest module documentation and add an entry in the What's New document.

My reasoning for not adding the change to the docs was that the pattern of use of test methods and the way they are described in the docs don't leave any reason for returning a value from a test method, so it seems like the only case when that would happen would be a mistake (possibly to be used by 3rd party code, but that seems unlikely).

But I don't see any harm in adding it to docs, see below. I'm curious if there are scenarios where users might want to return values from test method and this change to the docs would dissuade them.

I'm not sure if the docs should try to explain the reason for the change, it seems like a short explanation would be puzzling and a good explanation would be too long.

Proposed text:

Changed in 3.11:
  The behavior of returning a value from a test (other than the default None value), is now deprecated.

[akulakov]

I will look more into it, just posting here in case you know why it doesn't work in the same way...

I got it now, please ignore..

[serhiy-storchaka]

It is better to move this outside of the with block.

Suggested change

	self.assertIn('It is deprecated to return a value!=None', w.warnings[0].message.args[0])
	self.assertIn('test', w.warnings[0].message.args[0])
	self.assertIn(w.warnings[0].filename, __file__)
	self.assertIn('It is deprecated to return a value!=None', w.warnings[0].message.args[0])
	self.assertIn('test', w.warnings[0].message.args[0])
	self.assertIn(w.warnings[0].filename, __file__)

[serhiy-storchaka]

Not assertIn, but assertEqual.

[serhiy-storchaka]

'test' is too general name. This substring is a part of the warning message ("... from a test case ..."), so the test is passed in any case. Use more specific names, like test1, test2, test_func, test_gen.

Also, would not be better to test str(w.warnings[0].message) instead of w.warnings[0].message.args[0]?

[serhiy-storchaka]

assertEquals is a deprecated alias. Use assertEqual.

[akulakov]

Ah, somehow missed the deprecation warnings in a PR for deprecation warnings :) thanks for catching that.

[akulakov]

@serhiy-storchaka thanks for reviews! :)