Brian Skinn <bsk...@alum.mit.edu> added the comment:
It looks to me like it's a standard feature of the CPython string rendering
routines, where if single and double quotes are present in any string, the
preferred rendering is enclosure with single quotes with escaped internal
single quotes.
On 3.6.6, regardless how I enter the following, it always returns enclosed in
single quotes:
>>> """ ' " """
' \' " '
>>> ''' ' " '''
' \' " '
>>> ' \' " '
' \' " '
>>> " ' \" "
' \' " '
For my particular situation, then, the problem is that my warning message, as
it sits in the source, consists of a double-quoted string that contains single
quotes. Then, when 3.8 doctest goes to print the source line, it has to print
a string containing both single and double quotes, so the above default
rendering rule kicks in and it gets printed with enclosing single-quotes. For
3.7 doctest, where the regex doesn't match, the source line doesn't get
printed, and so the resulting string contains no double quotes, and thus the
string gets printed with enclosing double quotes.
Clearly, the solution is just for me to change the warning message! And indeed,
changing to `warnings.warn("foo has no bar")` and updating the expected result
to `'...UserWarning: foo has no bar\n...'` yields a passing test on both 3.7
and 3.8 now.
----------
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue36695>
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