Improve "Leading padding not allowed" error in Base32 and Base64 decoders

[serhiy-storchaka]

Feature or enhancement

Previously it only was raised if "=" occurs at the start of the Base64 encoded data in strict mode. Now it is also raised for Base32 (always strict mode). If invalid "=" occurs after complete group, we get "Excess padding not allowed".

But now we have better control on what errors to ignore, we can ignore whitespace characters without ignoring bad "=". I think that if "=" is only preceded by ignored characters, without any alphabetical character, "Leading padding not allowed" is more correct than "Excess padding not allowed".

>>> import base64
>>> base64.b64decode(b'=abcd', ignorechars=b' ')
binascii.Error: Leading padding not allowed
>>> base64.b64decode(b' =abcd', ignorechars=b' ')
binascii.Error: Excess padding not allowed

Should be:

>>> base64.b64decode(b' =abcd', ignorechars=b' ')
binascii.Error: Leading padding not allowed

Linked PRs

• gh-148016: Improve "Leading padding not allowed" error in Base32 and Base64 decoders #148017