If validate=True is passed to base64.b64decode(), it should raise a binascii.Error if the input contains any character not from the acceptable alphabet.But it does not raise if the input ends with a single \n. It raises if the input ends with a multiple \n or with any other whitespace character. Only a single \n is accepted.This is an implementation artifact. A regular exception ending with $ is used to validate an input. But $ matches not only end of string. It matches also an empty string before the trailing \n.Similar errors are also occurred in other sites. I'll open separate issues for different cases.

New changesetb19c0d77e6f25ea831ab608c71f15d0d9266c8c4 by Serhiy Storchaka in branch 'master':bpo-39055: Reject a trailing \n in base64.b64decode() with validate=True. (GH-17616)https://github.com/python/cpython/commit/b19c0d77e6f25ea831ab608c71f15d0d9266c8c4

New changeset3bdb2d9daf3ce41b325bd508e3dd29041e85dd1f by Miss Islington (bot) in branch '3.7':bpo-39055: Reject a trailing \n in base64.b64decode() with validate=True. (GH-17616)https://github.com/python/cpython/commit/3bdb2d9daf3ce41b325bd508e3dd29041e85dd1f

New changeset34aa3e71dc52c1a31336302905b9ac011a310412 by Miss Islington (bot) in branch '3.8':bpo-39055: Reject a trailing \n in base64.b64decode() with validate=True. (GH-17616)https://github.com/python/cpython/commit/34aa3e71dc52c1a31336302905b9ac011a310412