Redirect warning output from stderr to stdout in doctests

[tobiasdiez]

Pytest considers ouput in stderr as errors, thus we redirect the doctest warnings to stdout instead. Doesn't seem to make a difference for sage's doctest runner.

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[user202729]

Seems semantically incorrect because…

    INPUT: see :func:`warnings.showwarning`.

→

https://docs.python.org/3/library/warnings.html#warnings.showwarning

warnings.showwarning(message, category, filename, lineno, file=None, line=None)

    Write a warning to a file. The default implementation calls formatwarning(message, category, filename, lineno, line) and writes the resulting string to file, which defaults to [sys.stderr](https://docs.python.org/3/library/sys.html#sys.stderr).

How would programs that are tested using pytest normally test for warnings to be raised then?

[tobiasdiez]

Seems semantically incorrect because…

    INPUT: see :func:`warnings.showwarning`.

→

https://docs.python.org/3/library/warnings.html#warnings.showwarning

warnings.showwarning(message, category, filename, lineno, file=None, line=None)

    Write a warning to a file. The default implementation calls formatwarning(message, category, filename, lineno, line) and writes the resulting string to file, which defaults to [sys.stderr](https://docs.python.org/3/library/sys.html#sys.stderr).

Right, but in the context of doctest the distinction between stdout and stderr doesn't seem to be relevant (both are printed-out as strings by default).

How would programs that are tested using pytest normally test for warnings to be raised then?

In normal pytest methods, one can use the capsys fixture. But this doesn't work for doctests.

Sympy has a [warns context manager](https://docs.sympy.org/latest/modules/testing/pytest.html#sympy.testing.pytest.warns] that checks that a given warning is emitted. I think having something like this in place also for doctests would be helpful and would make the tests more robust. But this is something for another PR.

[user202729]

in the context of doctest

If this function is to be used nowhere outside doctest then I suppose this is acceptable. But I suggest modify the documentation for file= in the docstring then.

Is there really no other occurrence of stderr being used? This feels weird.

[tobiasdiez]

in the context of doctest

If this function is to be used nowhere outside doctest then I suppose this is acceptable.

Yes, it's part of doctest.forker and not used anywhere else.

But I suggest modify the documentation for file= in the docstring then.

Done.

Is there really no other occurrence of stderr being used? This feels weird.

Not that I could find.

[tobiasdiez]

@user202729 does this look good to you, or do you have further suggestions?