Incredibly super pedantic, but:

`'/'.join(('users', self.client, 'credentials'))` is faster/more efficient than using the +. You might also consider `os.path.join` or `urlparse.urljoin`.

References:
http://paolobernardi.wordpress.com/2012/11/06/python-string-concatenation-vs-list-join/
http://www.skymind.com/~ocrow/python_string/