DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats

[mgedmin]

What did you do?

Ran my test suite on Python 3.8.0a3

What did you expect to happen?

The test to pass

What actually happened?

The tests did pass, but there were new deprecation warnings from Pillow:

.../.tox/py38/lib/python3.8/site-packages/PIL/Image.py:477: DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats
    return encoder(mode, *args + extra)

These were introduced by https://bugs.python.org/issue36381 to warn about an upcoming Python C API change. The meaning of PY_SSIZE_T_CLEAN is described in https://python.readthedocs.io/en/stable/c-api/arg.html#strings-and-buffers.

What Pillow needs to do is:

• find all invocations of PyArg_Parse and related functions that use # formats
  make sure the type of the length argument is a Py_ssize_t
• #define PY_SSIZE_T_CLEAN above the #include <Python.h>.

PY_SSIZE_T_CLEAN was introduced in Python 2.5 so there shouldn't be any backwards-compatibility issues.

What are your OS, Python and Pillow versions?

• OS: Ubuntu 18.10
• Python: 3.8.0a3
• Pillow: 5.4.1

Reproduction

#!/usr/bin/env python3.8
import warnings
warnings.simplefilter("once")
from PIL import Image
Image.new("RGB", (1, 1)).save("/tmp/pixel.png")

[hugovk]

Please could you test with #3749?

[mgedmin]

Of course I did a search for existing issues and didn't think to check for existing pull requests.

It looks like the same issue, and the PR looks good to me (who is an absolute stranger to the Python C API).

How do I pip install a github PR into a virtualenv to test?

[mgedmin]

How do I pip install a github PR into a virtualenv to test?

What I did was clone the repo, fetch the pull/3749 ref, check it out, and .tox/py36/bin/pip install ~/src/Pillow.

The deprecation warning is gone and all my tests pass.

[hugovk]

Thanks for the report and testing, #3749 has been merged and will be in Monday's 6.0.0 release.