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)
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