code: def sign_request(self, signature_method, consumer, token):
          
  
          if not self.is_form_encoded:
              # according to
              # http://oauth.googlecode.com/svn/spec/ext/body_hash/1.0/oauth-bodyhash.html
              # section 4.1.1 "OAuth Consumers MUST NOT include an
              # oauth_body_hash parameter on requests with form-encoded
              # request bodies."
              if not self.body:
                 self.body = ''
              self['oauth_body_hash'] = base64.b64encode(sha1(to_utf8(self.body)).digest())
  
          if 'oauth_consumer_key' not in self:
              self['oauth_consumer_key'] = consumer.key
  
          if token and 'oauth_token' not in self:
              self['oauth_token'] = token.key
  
          self['oauth_signature_method'] = signature_method.name
          self['oauth_signature'] = signature_method.sign(self, consumer, token)
docs: Set the signature parameter to the result of sign.