linecache cannot get source for the __main__ module with a custom loader

[eltoder]

BPO	42125
Nosy	@ncoghlan, @vstinner, @eltoder, @FFY00
PRs	gh-86291: linecache: get module name from __spec__ if available #22908
Files	t.zip

^{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 2020-10-23.05:26:53.150>
labels = ['type-bug', 'library', '3.10']
title = 'linecache cannot get source for the __main__ module with a custom loader'
updated_at = <Date 2021-05-29.17:12:41.769>
user = 'https://github.com/eltoder'

bugs.python.org fields:

activity = <Date 2021-05-29.17:12:41.769>
actor = 'FFY00'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-10-23.05:26:53.150>
creator = 'eltoder'
dependencies = []
files = ['49536']
hgrepos = []
issue_num = 42125
keywords = ['patch']
message_count = 1.0
messages = ['379408']
nosy_count = 5.0
nosy_names = ['ncoghlan', 'vstinner', 'python-dev', 'eltoder', 'FFY00']
pr_nums = ['22908']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42125'
versions = ['Python 3.10']

Linked PRs

• [3.12] gh-86291: linecache: get module name from __spec__ if available (GH-22908) #115731
• [3.11] gh-86291: linecache: get module name from __spec__ if available (GH-22908) #115732

[eltoder]

If a module has a loader, linecache calls its get_source() passing __name__ as the argument. This works most of the time, except that the __main__ module has it set to "__main__", which is commonly not the real name of the module. Luckily, we now have __spec__ which has the real name, so we can just use it.

Attached zip file reproduces the problem:
$ python t.zip
Traceback (most recent call last):
  ...
  File "t.zip/t.py", line 11, in <module>
  File "t.zip/t.py", line 8, in f
  File "t.zip/t.py", line 8, in f
  File "t.zip/t.py", line 8, in f
  [Previous line repeated 2 more times]
  File "t.zip/t.py", line 7, in f
ValueError

Note that entries from t.py don't have source code lines.