doctest.DocTestFinder reports incorrect line numbers with exclude_empty=False

[cpitclaudel]

BPO	28249
Nosy	@cpitclaudel, @tirkarthi, @sobolevn, @kumaraditya303
PRs	bpo-28249: fix lineno location for empty DocTest instances #30498
Files	example.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-09-22.15:32:20.468>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'doctest.DocTestFinder reports incorrect line numbers with exclude_empty=False'
updated_at = <Date 2022-01-09.11:39:50.113>
user = 'https://github.com/cpitclaudel'

bugs.python.org fields:

activity = <Date 2022-01-09.11:39:50.113>
actor = 'kumaraditya'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-09-22.15:32:20.468>
creator = 'cpitclaudel'
dependencies = []
files = ['44786']
hgrepos = []
issue_num = 28249
keywords = ['patch']
message_count = 1.0
messages = ['277230']
nosy_count = 4.0
nosy_names = ['cpitclaudel', 'xtreak', 'sobolevn', 'kumaraditya']
pr_nums = ['30498']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28249'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[cpitclaudel]

Line numbers reported by the doctest module are wrong when a function does not include a docstring. With the attached example file, running

python -c "import doctest, example; print(doctest.DocTestFinder(exclude_empty=False).find(example))"

produces

[<DocTest example from example.py:5 (no examples)>,
 <DocTest example.a from example.py:5 (no examples)>,
 <DocTest example.b from example.py:5 (no examples)>,
 <DocTest example.c from example.py:None (no examples)>]

whereas if one uncomments the docstrings of a and c the output is

[<DocTest example from example.py:1 (no examples)>,
 <DocTest example.a from example.py:1 (no examples)>,
 <DocTest example.b from example.py:5 (no examples)>,
 <DocTest example.c from example.py:9 (no examples)>]

This bug is due to this line in doctest:

    lineno = self._find_lineno(obj, source_lines)

The documentation of _find_lineno says this:

    def _find_lineno(self, obj, source_lines):
        """
        Return a line number of the given object's docstring.  Note:
        this method assumes that the object has a docstring.
        """

This assumption is violated by the call listed above, because of the exclude_empty=False parameter to DocTestFinder().

I guess lineno should just be None for all methods that do not have a docstring?

[ambv]

This is now fixed.

[ambv]

Thanks, Clément!