On Mac/Linux, os.path.join is an alias for posixpath.join, which always joins path segments with /. On Windows, os.path.join is an alias for ntpath.join, which always uses \. When dealing with URLs, we always want forward slashes, regardless of platform, so posixpath.join should be used to build URL paths.

Running:

from __future__ import print_function

from six.moves.urllib_parse import urljoin as abs_urljoin
from posixpath import join as path_urljoin

def urljoin(site, path):
    return abs_urljoin(site, path)

def test_join(site, path):
    result = urljoin(site, path)
    print("'{0}' + '{1}'\n\t-> '{2}'".format(site, path, result))
    return result

local_path = path_urljoin('2016', '07', '12', 'release', 'index.html')

test_join('https://www.example.com', 'foo/bar/quux.js')
test_join('https://www.example.com', local_path)
test_join('https://www.example.com/', local_path)
test_join('https://www.example.com/prefix', local_path)

Yields:

'https://www.example.com' + 'foo/bar/quux.js'
        -> 'https://www.example.com/foo/bar/quux.js'
'https://www.example.com' + '2016/07/12/release/index.html'
        -> 'https://www.example.com/2016/07/12/release/index.html'
'https://www.example.com/' + '2016/07/12/release/index.html'
        -> 'https://www.example.com/2016/07/12/release/index.html'
'https://www.example.com/pre/fix' + '2016/07/12/release/index.html'
        -> 'https://www.example.com/2016/07/12/release/index.html'

The last one is a problem, as the path after the hostname, /pre/fix, is lost.

We can fix it using urlparse to separate that path from the hostname:

from six.moves.urllib_parse import urljoin as abs_urljoin, urlparse

def urljoin(site, path):
    return abs_urljoin(site, path_urljoin(urlparse(site).path, path))

which yields:

'https://www.example.com' + 'foo/bar/quux.js'
        -> 'https://www.example.com/foo/bar/quux.js'
'https://www.example.com' + '2016/07/12/release/index.html'
        -> 'https://www.example.com/2016/07/12/release/index.html'
'https://www.example.com/' + '2016/07/12/release/index.html'
        -> 'https://www.example.com/2016/07/12/release/index.html'
'https://www.example.com/pre/fix' + '2016/07/12/release/index.html'
        -> 'https://www.example.com/pre/fix/2016/07/12/release/index.html'

Note that if the path argument to my urljoin function is absolute, any path suffix to site is discarded:

'https://www.example.com/pre/fix' + '/abs/path'
        -> 'https://www.example.com/abs/path'

That in turn can be fixed by using str.split('/') and discarding any empty segments.

def urljoin(site, path):
    if isinstance(path, six.string_types):
        segments = [s for s in path.split('/') if s]
    else:
        segments = path  # assume list or tuple
    return abs_urljoin(site, path_urljoin(urlparse(site).path, *segments))

Thus:

'https://www.example.com/pre/fix' + '/abs/path'
    -> 'https://www.example.com/pre/fix/abs/path'
'https://www.example.com/pre/fix' + '('one', 'two', 'three')'
    -> 'https://www.example.com/pre/fix/one/two/three'