Here's a simple test case that shows the bug:

# python
Python 2.7.9 (default, Apr  2 2015, 14:49:18) 
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import StringIO, tokenize
>>> code = "if False:\n\tx=3\n \ty=3\n"
>>> g = list(tokenize.generate_tokens(StringIO.StringIO(code).readline))
>>> assert tokenize.untokenize(g) == code
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError


This is very similar to: https://bugs.python.org/issue20387

I'm using the patch that was provided to fix that bug, but with the patch this slightly different test case still fails. On the last line of "code" from my example, where the line begins with a single space then a tab, the untokenized string reverses the order, and the tab precedes the space. This generates code that will throw an IndentationError if run.

Python 2.7 is end-of-life, see e.g. https://www.python.org/doc/sunset-python-2/ so this is unlikely to be fixed.

This behaviour continues on Python 3 it seems (I used 3.8.1), *however* on Python 3 this is not valid code as it mixes spaces and tabs, see e.g. https://www.python.org/dev/peps/pep-0008/#tabs-or-spaces

I think therefore this can be closed, though I might well be mistaken.

Python 2.7 is no longer supported.