PyUnicode_FindChar declares in the doc it treats its *start* and *end* parameters as str[start:end], same as other APIs like PyUnicode_Find, PyUnicode_Count. But it doesn't allow negative indices like others so violates the doc.

PyUnicode_FindChar.patch is a new feature, it cannot be applied to stable branches (py < 3.7).

I'm not sure that it's worth it to support negative indexes for end. Why not simply documenting that end must be positive?

Other APIs like PyUnicode_Find and PyUnicode_Count support it. Their docs are almost the same so I think PyUnicode_FindChar does not need to be the special one. After change, its behaviour and implementation are more consistent with other APIs.

Serhiy: I don't think that it's worth it to add a new function to _testcapi to test PyUnicode_FindChar. The implementation of the function seems simple.

At least, I would prefer to only see a few unit tests, not 17 test for this simple function!

I mean "character in str" is already tested by a *lot* of unit tests.

I think it is nice to add tests for C API. Especially if there is no direct mapping between Python and C API ("character in str" don't call PyUnicode_FindChar()). Tests should cover all corner cases, otherwise we can miss bugs. Some C API can be not used in CPython at all, just in third-party extensions, and special tests is the only way to test them. The implementation of PyUnicode_FindChar() is not so simple (for example see issue24821).

I don't have an opinion about supporting negative indices.

Would be nice to test corner cases:

1. Search UCS2 or UCS4 character with zero lower 8 bits: U+XX00.

2. Search UCS2 or UCS4 character with lower 8 bits that match high bits of string characters. For example search U+0404 in the string that consists of U+04XX (Ukrainian text). I think you can find similar Chinese example.

Thanks for your reviews. :-)

v2 updated the test codes.

PyUnicode_FindChar-v2.patch LGTM with a minor comment on the review, but I would prefer that Serhiy also reviews it ;-)

Remaining question: what is the behaviour for direction=0, direction=100 or direction=-2? Maybe we can add a few unit tests for strange values of direction? (Not sure if it's worth it.)

> Remaining question: what is the behaviour for direction=0, direction=100 or direction=-2? Maybe we can add a few unit tests for strange values of direction? (Not sure if it's worth it.)

It's not documented so I also doubt it. Expect Serhiy's comment.

Ignore my request about special direction values. It's not worth it to writ tests for that.

PyUnicode_FindChar-v2.patch LGTM.

New changeset ce6a6cc3765d by Xiang Zhang in branch 'default':
Issue #28822: Adjust indices handling of PyUnicode_FindChar().
https://hg.python.org/cpython/rev/ce6a6cc3765d

Thanks Victor and Serhiy!

New changeset 2b5e5a3a805e by Martin Panter in branch 'default':
Issue #28822: Add susp-ignored entry for NEWS; fix grammar
https://hg.python.org/cpython/rev/2b5e5a3a805e