The doctoring says "If some location information (lineno, end_lineno, col_offset, or end_col_offset) is missing, return None."

However:

>>> s = "12"
>>> node = ast.parse(s).body[0]
>>> ast.get_source_segment(s, node)
'12'
>>> del node.lineno
>>> ast.get_source_segment(s, node)
>>> node = ast.parse(s).body[0]
>>> del node.end_lineno
>>> ast.get_source_segment(s, node)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/iritkatriel/src/cpython/Lib/ast.py", line 336, in get_source_segment
    end_lineno = node.end_lineno - 1
TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'

In other parts of ast.py there are hasattr checks for lineno/col_offset alongside checks for None for end_lineno/end_col_offset. So this difference in behaviour of the end_ fields seems intended.

The code works on 3.8 for me, but has regressed on 3.9 master. Looks like this is caused by https://bugs.python.org/issue36287 (https://github.com/python/cpython/pull/18843)

New changeset e6578a226d8a8a13d1062d154fad0fef28ee2416 by Irit Katriel in branch 'master':
bpo-40662: Fixed ast.get_source_segment for ast nodes that have incomplete location information (GH-20157)
https://github.com/python/cpython/commit/e6578a226d8a8a13d1062d154fad0fef28ee2416