code: def _get_signature_method(self, request):
          
          signature_method = request.get('oauth_signature_method')
          if signature_method is None:
              signature_method = SIGNATURE_METHOD
  
          try:
              # Get the signature method object.
              return self.signature_methods[signature_method]
          except KeyError:
              signature_method_names = ', '.join(self.signature_methods.keys())
              raise Error('Signature method %s not supported try one of the '
                          'following: %s'
                              % (signature_method, signature_method_names))
docs: Figure out the signature with some defaults.