Issue 35798: duplicate SyntaxWarning: "is" with a literal

$ python3.8 -c 'if object() is 42: pass'
<string>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
<string>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?

I'd like only one copy of this warning, not two.

Tested with git master (e9b185f2a493cc54f0d49eac44bf21e8d7de2990).

The warning has been introduced by bpo-34850: commit 3bcbedc9f1471d957a30a90f9d1251516b422416.

can I work on this?

I verified that master on Windows (which requires " instead of ')
> python -c "if object() is 42: pass"
results in the doubled messsage, and that after applying PR 11639 and recompiling, there is only 1 message.

We should test that exactly 1 warning is emitted.  The following fails on master and passes with the parch:

import unittest, warnings

class SyntaxWarningTest(unittest.TestCase):
    def test_syntax_warning_once(self):
        with warnings.catch_warnings(record=True) as w:
            warnings.simplefilter("always")
            compile('if object() is 42: pass\n', '', 'single')
            self.assertEqual(len(w), 1)  # Not 2, see issue 35798

if __name__ == '__main__':
    unittest.main()
    
The original patch added test_comparison_is_literal() in test_grammar.  The 'with' block above could be added at the end.

This warning is not special. I'll add a helper for testing that all syntax warnings are emitted only once later.

New changeset 4583525835baf8fc7bd49a60725d1e8c49ef92b3 by Serhiy Storchaka in branch 'master':
bpo-35798: Fix duplicate SyntaxWarning: "is" with a literal. (GH-11639)
https://github.com/python/cpython/commit/4583525835baf8fc7bd49a60725d1e8c49ef92b3

New changeset e7a4bb554edb72fc6619d23241d59162d06f249a by Serhiy Storchaka in branch 'master':
bpo-35798: Add test.support.check_syntax_warning(). (#11895)
https://github.com/python/cpython/commit/e7a4bb554edb72fc6619d23241d59162d06f249a

messages: + msg335670