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

[Brian Skinn]

New submission from Brian Skinn <bskinn at alum.mit.edu>:

In [this project](https://github.com/bskinn/stdio-mgr) of mine, I have a tox matrix set up with Pythons from 3.3. to 3.8. I have pytest set up to run doctest on my [`README.rst`](https://github.com/bskinn/stdio-mgr/blob/6444cce8e5866e2d519c1c0630551d8867f30c9a/README.rst).  For Pythons 3.4 to 3.7 (3.4.10, 3.5.7, 3.6.8, 3.7.2), the following doctest example passes:

```
>>> import warnings
>>> with stdio_mgr() as (in_, out_, err_):
...     warnings.warn("'foo' has no 'bar'")
...     err_cap = err_.getvalue()
>>> err_cap
"...UserWarning: 'foo' has no 'bar'\n..."
```

Under Python 3.8.0a3, though, it fails (actual local paths elided):

```
$ tox -re py38-attrs_latest
.package recreate: .../.tox/.package
.package installdeps: wheel, setuptools, attrs>=17.1
py38-attrs_latest recreate: .../.tox/py38-attrs_latest
py38-attrs_latest installdeps: attrs, pytest
py38-attrs_latest inst: .../.tox/.tmp/package/1/stdio-mgr-1.0.2.dev1.tar.gz
py38-attrs_latest installed: atomicwrites==1.3.0,attrs==19.1.0,more-itertools==7.0.0,pluggy==0.9.0,py==1.8.0,pytest==4.4.1,six==1.12.0,stdio-mgr==1.0.2.dev1
py38-attrs_latest run-test-pre: PYTHONHASHSEED='2720295779'
py38-attrs_latest run-test: commands[0] | pytest
=============================================================================================== test session starts ================================================================================================
platform linux -- Python 3.8.0a3, pytest-4.4.1, py-1.8.0, pluggy-0.9.0
cachedir: .tox/py38-attrs_latest/.pytest_cache
rootdir: ..., inifile: tox.ini
collected 6 items                                                                                                                                                                                                  

README.rst F                                                                                                                                                                                                 [ 16%]
tests/test_stdiomgr_base.py .....                                                                                                                                                                            [100%]

===================================================================================================== FAILURES =====================================================================================================
_______________________________________________________________________________________________ [doctest] README.rst _______________________________________________________________________________________________
077 
078 **Mock** ``stderr``\ **:**
079 
080 .. code ::
081 
082     >>> import warnings
083     >>> with stdio_mgr() as (in_, out_, err_):
084     ...     warnings.warn("'foo' has no 'bar'")
085     ...     err_cap = err_.getvalue()
086     >>> err_cap
Expected:
    "...UserWarning: 'foo' has no 'bar'\n..."
Got:
    '<doctest README.rst[4]>:2: UserWarning: \'foo\' has no \'bar\'\n  warnings.warn("\'foo\' has no \'bar\'")\n'

.../README.rst:86: DocTestFailure
======================================================================================== 1 failed, 5 passed in 0.06 seconds ========================================================================================
ERROR: InvocationError for command .../.tox/py38-attrs_latest/bin/pytest (exited with code 1)
_____________________________________________________________________________________________________ summary ______________________________________________________________________________________________________
ERROR:   py38-attrs_latest: commands failed

```

If I change the doctest in README to the following, where the expected output is surrounded by single-quotes instead of double-quotes, and the internal single quotes are escaped, it passes fine in 3.8.0a3:

```
>>> import warnings
    >>> with stdio_mgr() as (in_, out_, err_):
    ...     warnings.warn("'foo' has no 'bar'")
    ...     err_cap = err_.getvalue()
    >>> err_cap
    '...UserWarning: \'foo\' has no \'bar\'\n...'
```

But, naturally, it fails in 3.7 and below.

It *looks* like this is probably a glitch somewhere in 3.8.0a3, where this string containing single quotes is rendered (at the REPL?) using enclosing single quotes and escaped internal single quotes, rather than enclosing double-quotes and non-escaped internal single-quotes?

----------
components: Library (Lib)
messages: 340637
nosy: bskinn
priority: normal
severity: normal
status: open
title: Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning
type: behavior
versions: Python 3.8

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36695>
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