urlparse implements RFC 1808.  That is strongly out of
date.  The most recent is RFC 3986.

Here is a text from 4Suite

    # Reasons to avoid using urllib.basejoin() and
urlparse.urljoin():
    # - Both are partial implementations of
long-obsolete specs.
    # - Both accept relative URLs as the base, which no
spec allows.
    # - urllib.basejoin() mishandles the '' and '..'
references.
    # - If the base URL uses a non-hierarchical or
relative path,
    #    or if the URL scheme is unrecognized, the
result is not
    #    always as expected (partly due to issues in
RFC 1808).
    # - If the authority component of a 'file' URI is
empty,
    #    the authority component is removed altogether.
If it was
    #    not present, an empty authority component is
in the result.
    # - '.' and '..' segments are not always collapsed
as well as they
    #    should be (partly due to issues in RFC 1808).
    # - Effective Python 2.4, urllib.basejoin() *is*
urlparse.urljoin(),
    #    but urlparse.urljoin() is still based on RFC 1808.

See also the back python-dev discussions on "urlparse"
for examples of people wanting a better/more up-to-date
urlparse/urljoin.

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See also bug 1462525 which has a 'uriparse.py' replacement
for urlparse, claiming better compliance.