When given an invalid base64 string that starts with a valid base64 substring, the functions will return the decoded bytes only up to the substring rather then ignoring the non-alphabet character.

Examples:
>>> base64.b64decode("AAAAAAAA")
b'\x00\x00\x00\x00\x00\x00'
>>> base64.b64decode("AA=AAAAAA")
b'\x00\x00\x00\x00\x00\x00'
>>> base64.b64decode("AAA=AAAAA")
b'\x00\x00'

I am not sure if simply ignoring the non-valid character is the best way to go. Feels like silencing errors.

b64decode does accept the 'validate' flag - defaulted to False - that will halt the execution and throw an error.

What might be a good idea is to implement an 'errors' argument that accepts 'ignore' as a value, like we do for bytes.decode (https://docs.python.org/3/library/stdtypes.html#bytes.decode)

Actually, I'm not even sure if it makes sense to decode the 'first valid substring'... IMHO, we should warn the user

For reference in future discussions, Python's base64 module implements RFC 3548 (https://tools.ietf.org/html/rfc3548) whose section 2.3 (https://tools.ietf.org/html/rfc3548#section-2.3) discusses about "Interpretation of non-alphabet characters in encoded data".

The section's content is:

   Base encodings use a specific, reduced, alphabet to encode binary
   data.  Non alphabet characters could exist within base encoded data,
   caused by data corruption or by design.  Non alphabet characters may
   be exploited as a "covert channel", where non-protocol data can be
   sent for nefarious purposes.  Non alphabet characters might also be
   sent in order to exploit implementation errors leading to, e.g.,
   buffer overflow attacks.

   Implementations MUST reject the encoding if it contains characters
   outside the base alphabet when interpreting base encoded data, unless
   the specification referring to this document explicitly states
   otherwise.  Such specifications may, as MIME does, instead state that
   characters outside the base encoding alphabet should simply be
   ignored when interpreting data ("be liberal in what you accept").
   Note that this means that any CRLF constitute "non alphabet
   characters" and are ignored.  Furthermore, such specifications may
   consider the pad character, "=", as not part of the base alphabet
   until the end of the string.  If more than the allowed number of pad
   characters are found at the end of the string, e.g., a base 64 string
   terminated with "===", the excess pad characters could be ignored.

In my opinion, the RFC is rather permissive about strange characters in the encoded data. The RFC refers to the MIME specification that ignores the data and hints the possibility of rejecting the pad symbol '=' unless it is found in the end of the string.

I think that our best option if we would like to address this issue is to add an 'errors' argument whose default value will keep the current behavior for backwards compatibility but will accept more options in order to both ignore the strange characters and carry on with the processing - like bytes.decode's errors=ignore flag - and to raise an error in such situations, like bytes.decode's errors=strict.