http://bugs.python.org/issue1669539 has been partially fixed. On Windows os.path.join('foo', 'a:bar') gives 'a:bar' not 'foo\\a:bar'. However os.path.isabs('a:bar') returns False yet it causes a reset in the join like an absolute path.

'\foo' is considered an absolute path even though calling os.path.abspath on it can yield different results - as if it were a relative path. At minimum we should amend the wording about what resets the join.