[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn <bskinn at alum.mit.edu> added the comment:

<nod>, it seems like the problem must somehow stem from the new commit using frame.f_code.co_filename (or the C equivalent), instead of using __file__ as previously.

Consider this warn2.py, similar to the other but with no single quotes in the warning message:

```
import doctest

class Tester:
    r"""Provide docstring for testing.

    >>> import warnings
    >>> from contextlib import redirect_stderr
    >>> from io import StringIO
    >>> sio = StringIO()
    >>> with redirect_stderr(sio):
    ...     warnings.warn("Warning with no quotes")
    >>> sio.getvalue()
    '...'

    """

doctest.run_docstring_examples(
    Tester(),
    {},
    optionflags=doctest.ELLIPSIS,
)
```

This doctest PASSES for me in both 3.7 and 3.8; note that the expected doctest output from `sio.getvalue()` is *single-quote delimited*. This implies to me that REPL string output is single-quote delimited by default, and that there's some sort of "smart string formatter" functionality involved that is working correctly in 3.7 but not in 3.8, which REPL-prints the single-quote-containing string using enclosing double-quotes, so as to avoid escaping the internal single quotes.

Why 11a8966 would break this in this way is ... baffling to me.

---

Unfortunately, I don't think it will work to fix the doctest on my end simply by using `print(sio.getvalue())`, as the resulting message is one line long in 3.7, but two lines long in 3.8. Further, doctest gets confused when you try to start a line of output with an ellipsis, as it thinks it's a continuation of the prior command.

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<https://bugs.python.org/issue36695>
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