code: def get_mapping_function(function_name, functions_mapping):
      
      if function_name in functions_mapping:
          return functions_mapping[function_name]
  
      elif function_name in ["parameterize", "P"]:
          from httprunner import loader
          return loader.load_csv_file
  
      elif function_name in ["environ", "ENV"]:
          return utils.get_os_environ
  
      try:
          # check if HttpRunner builtin functions
          from httprunner import loader
          built_in_functions = loader.load_builtin_functions()
          return built_in_functions[function_name]
      except KeyError:
          pass
  
      try:
          # check if Python builtin functions
          item_func = eval(function_name)
          if callable(item_func):
              # is builtin function
              return item_func
      except (NameError, TypeError):
          # is not builtin function
          raise exceptions.FunctionNotFound("{} is not found.".format(function_name))
docs: get function from functions_mapping,
          if not found, then try to check if builtin function.
  
      Args:
          variable_name (str): variable name
          variables_mapping (dict): variables mapping
  
      Returns:
          mapping function object.
  
      Raises:
          exceptions.FunctionNotFound: function is neither defined in debugtalk.py nor builtin.