code: def _normalize_coerce(self, mapping, schema):
          
  
          error = errors.COERCION_FAILED
          for field in mapping:
              if field in schema and 'coerce' in schema[field]:
                  mapping[field] = self.__normalize_coerce(
                      schema[field]['coerce'], field, mapping[field],
                      schema[field].get('nullable', False), error)
              elif isinstance(self.allow_unknown, Mapping) and \
                      'coerce' in self.allow_unknown:
                  mapping[field] = self.__normalize_coerce(
                      self.allow_unknown['coerce'], field, mapping[field],
                      self.allow_unknown.get('nullable', False), error)
docs: {'oneof': [
                  {'type': 'callable'},
                  {'type': 'list',
                   'schema': {'oneof': [{'type': 'callable'},
                                        {'type': 'string'}]}},
                  {'type': 'string'}
                  ]}