gh-144001: Support ignorechars in binascii.a2b_base64() and base64.b64decode()

[serhiy-storchaka]

• Issue: Add the ignorechars parameter in the Base64 decoder #144001

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📚 Documentation preview 📚: https://cpython-previews--144024.org.readthedocs.build/

[serhiy-storchaka]

This is a draft because I am going to apply the error handling changes in a separate issue first.

We can also apply some optimizations -- cache the ignored chars and returning on fast path after ignored characters.

[serhiy-storchaka]

Well, the issue with error handling was just that the error message could be more specific in some cases. It is not a bug. It can be included in this PR.

[moneebullah25]

No worries, that was the point I was just free on Sunday, so I push out a PR in case you haven't started on it. BTW your PR looks good, I am no maintainer but you just have to run ./python Tools/build/generate_global_objects.py to generate the updated header files. I hope it helps.

[vstinner]

LGTM.

I'm fine with changing validate default to True if ignorechars is specified. The change produces more precise error messages. It also adds a lot of tests which is always a good thing :-)

[gpshead]

Taking a quick look: The docs in this PR changed the default for validate and strict_mode from False to True. But versionchanged does not mention that change and there was no deprecation period. Did this behavior change? The docs need fixing at a minimum but if the behavior changed we also need a deprecation cycle. What I understand @vstinner above to be saying is to change the default only in the case of the new parameter being supplied?

[gpshead]

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.