In the ast module, a function named _pad_whitespace has a doc string with escape sequences of \f and \t.

The current doc string from Lib/ast.py:305 is:

    """Replace all chars except '\f\t' in a line with spaces."""

Example of doc string output in a REPL:

    Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import ast
    >>> import inspect
    >>> inspect.getdoc(ast._pad_whitespace)
    "Replace all chars except '\x0c     ' in a line with spaces."
    >>>

The \x0c is the formfeed and the '     ' (5 spaces) was the tab.

It is my understanding that the output should be:

    "Replace all chars except '\f\t' in a line with spaces."

I would expect the source to be:

    """Replace all chars except '\\f\\t' in a line with spaces."""

or perhaps a raw string:

    r"""Replace all chars except '\f\t' in a line with spaces."""

The current Lib/ast.py:305 is Python 3.9.0 alpha 3 though the issue is also in Python 3.8.0 and 3.8.1 with Lib/ast.py:227 .
Python 3.7.4 Lib/ast.py does not have the function _pad_whitespace as it appears major code changes occurred in the ast module with Python 3.8.0.

Correction: Python 3.8.0 and 3.8.1 with Lib/ast.py:277

Line 227 is invalid and 277 is correct.

Correction: Python 3.8.0 and 3.8.1 with Lib/ast.py:277

Line 227 is invalid and 277 is correct and forgot to add 3.8 prefix. Sorry.

I think this would be an improvement, and an good issue for a newcomer.

I'd vote for the r-string, but it doesn't really matter.

But in both cases the 'Output' will contain '\\':
"Replace all chars except '\\f\\t' in a line with spaces."

I have submitted 2 Pull Requests. One for 3.9 and the other for 3.8. Only way I knew was to create 2 separate branches to checkout, linked to version tags to get the correct patches for ast.py. Hope is OK.

New changeset fbeba8f2481411d608a616366394e07cdc52e0bb by mpheath in branch 'master':
bpo-39524: Fixed doc-string in ast._pad_whitespace (GH-18340)
https://github.com/python/cpython/commit/fbeba8f2481411d608a616366394e07cdc52e0bb

The backport of the patch to 3.8 has failed.

New changeset f3982d666c1df290373a4810edd501b2176b45c7 by Irit Katriel in branch '3.8':
[3.8] bpo-39524: Fixed doc-string in ast._pad_whitespace (GH-18340) (GH-22857)
https://github.com/python/cpython/commit/f3982d666c1df290373a4810edd501b2176b45c7