I did some more debugging. doctest patches linecache which does some regex matching when filename is of the form <doctest <filename>[examplenumber]> to return example source. Before the commit seems absolute path was present in warning and hence this regex didn't match. With the commit returning the filename of this format that matches the regex the example line is again returned. This happens with warnings inside doctest because doctest patches linecache which is used by warnings.py during formatting the warning. In CPython for some reason presence of both single quote and double quote inside a triple quoted string causes the single quote to be escaped. Any concatenation with the escaped triple quoted string also escapes the resulting text. doctest seems to store the examples as single quoted strings that are escaped and escaping them during _formatwarnmsg_impl causes the other one also to be escaped. It also happens with a normal string that has an escaped double quote.

>>> a = """Test '' b""" # Two single quotes
>>> a
"Test '' b"
>>> a = """Test " b'""" # One single and double quote
>>> a
'Test " b\''
>>> a + "'c'"
'Test " b\'\'c\''
>>> a = """Test ' b"""  # Only single quote
>>> a
"Test ' b"
>>>> a + "'c'"
"Test ' b'c'"
>>>> a = "Test ' b\""  # Escaped double quote
>>>> a
'Test \' b"'
>>>> a + "'a'"
'Test \' b"\'a\''

Does anyone know why this happens with escaped quotes and single quote being escaped? Is this expected and is it part of spec about how single and double quote are swapped over representation?

Longer explanation :

Take the below sample doctest file

$ cat ../backups/bpo36695.rst
>>> import warnings  # line 0
>>> warnings.warn("Test 'a'")  # line 1

doctest patches linecache.getlines to a custom function `__patched_linecache_getlines` [0]

linecache.getlines = __patched_linecache_getlines

__LINECACHE_FILENAME_RE = re.compile(r'<doctest '
                                     r'(?P<name>.+)'
                                     r'\[(?P<examplenum>\d+)\]>$')
def __patched_linecache_getlines(self, filename, module_globals=None):
     m = self.__LINECACHE_FILENAME_RE.match(filename)
     if m and m.group('name') == self.test.name:
          example = self.test.examples[int(m.group('examplenum'))]
          return example.source.splitlines(keepends=True)
     else:
          return self.save_linecache_getlines(filename, module_globals)

doctest forms a special filename as below that is passed to exec(compile()) and hence as per the commit warning is now raised as the filename "<doctest bpo36695.rst[1]>" in the warning. doctest also mocks sys.stdout internally to have the output captured to a StringIO buffer. [1]

# Use a special filename for compile(), so we can retrieve
# the source code during interactive debugging (see
# __patched_linecache_getlines).
filename = '<doctest %s[%d]>' % (test.name, examplenum)

# Before commit

   cpython git:(3b0b90c8c3) ./python.exe -m doctest ../backups/bpo36695.rst
/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/doctest.py:1: UserWarning: Test 'a'
  # Module doctest.

# After commit

$    cpython git:(11a896652e) ./python.exe -m doctest ../backups/bpo36695.rst
<doctest bpo36695.rst[1]>:1: UserWarning: Test 'a'
  warnings.warn("Test 'a'")

formatting warning message [2] calls linecache.getline with filename as "<doctest bpo36695.rst[1]>" after commit which in turn calls linecache.getlines that is patched above by doctest and hence it matches the regex and returns the example.source "warnings.warn("Test 'a'")". It seems to be a triple quoted string that is already escaped and hence in the below line calling s += " %s\n" % line causes the actual warning message and the example source line to be escaped.

  def _formatwarnmsg_impl(msg):
    s =  ("%s:%s: %s: %s\n"
          % (msg.filename, msg.lineno, msg.category.__name__,
             msg.message))

    if msg.line is None:
        try:
            import linecache
            line = linecache.getline(msg.filename, msg.lineno)
        except Exception:
            # When a warning is logged during Python shutdown, linecache
            # and the import machinery don't work anymore
            line = None
            linecache = None
    else:
        line = msg.line
    if line:
        line = line.strip()
        s += "  %s\n" % line

[0] https://github.com/python/cpython/blob/29d018aa63b72161cfc67602dc3dbd386272da64/Lib/doctest.py#L1468
[1] https://github.com/python/cpython/blob/29d018aa63b72161cfc67602dc3dbd386272da64/Lib/doctest.py#L1452
[2] https://github.com/python/cpython/blob/29d018aa63b72161cfc67602dc3dbd386272da64/Lib/warnings.py#L35