docs: def get_schema_type(cls, schema):
          """ Get schema type for the argument
  
          :param schema: Schema to analyze
          :return: COMPILED_TYPE constant
          :rtype: str|None
          """
          schema_type = type(schema)
  
          # Marker
          if issubclass(schema_type, markers.Marker):
              return const.COMPILED_TYPE.MARKER
          # Marker Type
          elif issubclass(schema_type, six.class_types) and issubclass(schema, markers.Marker):
              return const.COMPILED_TYPE.MARKER
          # CompiledSchema
          elif isinstance(schema, CompiledSchema):
              return const.COMPILED_TYPE.SCHEMA
          else:
              return primitive_type(schema)