According to the documentation of base64 (https://docs.python.org/3/library/base64.html),

> Optional altchars must be a bytes-like object of at least length 2 (additional characters are ignored) which specifies an alternative alphabet for the + and / characters.

but this explanation is out-dated.

Actually this had been correct until the commit `4581ae5fa2450db3f00384e4b2e86654605100d4` was made for cpython in "2007".
(Thus, the explanation has been incorrect for about 15 years.)

The current implementation requires the length of `altchars` to be exactly two, as explicitly written in the comment:

> Optional altchars should be a byte string of length 2 which specifies an alternative alphabet for the '+' and '/' characters.

and the corresponding assertion is:

> assert len(altchars) == 2, repr(altchars)