gh-144001: Support ignorechars in binascii.a2b_base64() and base64.b64decode() by serhiy-storchaka · Pull Request #144024 · python/cpython

that may just be a simple doc fix. the declaration lines need to not state a value for the parameters who's default behavior varies based on context:

.. function:: b64decode(s, altchars=None, validate=False)
              b64decode(s, altchars=None, validate=True, *, ignorechars)

.. function:: a2b_base64(string, /, *, strict_mode=False)
              a2b_base64(string, /, *, strict_mode=True, ignorechars)

The text below describes a dynamic behavior on both, but people will absolutely misread the function signature and not read the text if we do not indicate it in the signature.