Incorrect documentation for s# arguments in C API argument parsing

[enrico]

BPO	37465
Nosy	@methane
PRs	bpo-37465: Parsing: all of s/y/z take a Py_ssize_t #17478

^{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 = <Date 2020-04-23.05:20:33.359>
created_at = <Date 2019-07-01.08:20:20.624>
labels = ['easy', '3.7', '3.8', '3.9', 'expert-C-API', 'type-feature', 'docs']
title = 'Incorrect documentation for `s#` arguments in C API argument parsing'
updated_at = <Date 2020-04-23.05:20:33.358>
user = 'https://bugs.python.org/enrico'

bugs.python.org fields:

activity = <Date 2020-04-23.05:20:33.358>
actor = 'methane'
assignee = 'docs@python'
closed = True
closed_date = <Date 2020-04-23.05:20:33.359>
closer = 'methane'
components = ['Documentation', 'C API']
creation = <Date 2019-07-01.08:20:20.624>
creator = 'enrico'
dependencies = []
files = []
hgrepos = []
issue_num = 37465
keywords = ['patch', 'easy']
message_count = 5.0
messages = ['346974', '346976', '346982', '347102', '367078']
nosy_count = 3.0
nosy_names = ['methane', 'docs@python', 'enrico']
pr_nums = ['17478']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue37465'
versions = ['Python 2.7', 'Python 3.7', 'Python 3.8', 'Python 3.9']

[enrico]

In https://docs.python.org/3.9/c-api/arg.html, in the documentation for parsing argument, there is:

 s# (str, read-only bytes-like object) [const char *, int or Py_ssize_t]

In my amd64 system, Py_ssize_t is a different type than int, and passing a Py_ssize_t causes undefine behaviour.

I assume this has been switched to an int in the API, and that thisinstance of the documentation has not been updated accordingly. At the bottom of the page in the documentation of Py_BuildValue, s# is correctly documented as using an int and no Py_ssize_t, for example.

[methane]

See note in https://docs.python.org/3.9/c-api/arg.html#strings-and-buffers

"""
Note: For all # variants of formats (s#, y#, etc.), the type of the length argument (int or Py_ssize_t) is controlled by defining the macro PY_SSIZE_T_CLEAN before including Python.h. If the macro was defined, length is a Py_ssize_t rather than an int. This behavior will change in a future Python version to only support Py_ssize_t and drop int support. It is best to always define PY_SSIZE_T_CLEAN.
"""

[enrico]

Oh! Fair enough, I had missed it. Does the note also involve Py_BuildValue? If so, the documentation of Py_BuildValue should probably be updated; if not, I think it would be clearer if the note mentioned that it only applies to parsing, not building, values.

[methane]

Oh! Fair enough, I had missed it. Does the note also involve Py_BuildValue? If so, the documentation of Py_BuildValue should probably be updated; if not, I think it would be clearer if the note mentioned that it only applies to parsing, not building, values.

Yes, this is same to Py_BuildValue. The document of Py_BuildValue
should be int or Py_ssize_t too.

[methane]

Fixed via #62863.