def __init__(self, key, context, **kwargs):
self.key = key<EOL>self.uri_format = '<STR_LIT>' + '<STR_LIT::>'.join(key)<EOL>self.context = context<EOL>
:param list key:
  provides list of string used to specify the rate limit
  bucket. for instance:

  >>> [
    'docido',
    '{service}',
    '{http_headers[user-agent]'
  ]
  >>>

:param dict context:
    provide part of the format required to explicit the key.
    for instance:

    >>> dict(service='twitter')
    >>>