I'm changing the name to better describe the problem, and suggest a better solution.

The urlparse.urlsplit and .urlunsplit functions currently don't validate the scheme argument, if given. According to the RFC:

   Scheme names consist of a sequence of characters. The lower case
   letters "a"--"z", digits, and the characters plus ("+"), period
   ("."), and hyphen ("-") are allowed. For resiliency, programs
   interpreting URLs should treat upper case letters as equivalent to
   lower case in scheme names (e.g., allow "HTTP" as well as "http").

https://www.ietf.org/rfc/rfc1738.txt

If the scheme is specified, I suggest it should be normalised to lowercase and validated, something like this:

    # untested
    if scheme:
        # scheme_chars already defined in module
        badchars = set(scheme) - set(scheme_chars)
        if badchars:
            raise ValueError('"%c" is invalid in URL schemes' % badchars.pop())
        scheme = scheme.lower()


This will help avoid errors such as passing 'http://' as the scheme.