urllib.parse has the following information in this module.

```
# A classification of schemes ('' means apply by default)
uses_relative = ['ftp', 'http', 'gopher', 'nntp', 'imap',
                 'wais', 'file', 'https', 'shttp', 'mms',
                 'prospero', 'rtsp', 'rtspu', '', 'sftp',
                 'svn', 'svn+ssh', 'ws', 'wss']

```

Note the '' in the list.

1) First it needs to be first one for easy identification.
2) It needs to be clarified. '' means apply by default does not help the reader.