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文件 docstrings

• 文件字串是一種註釋，用於解釋函式的作用以及使用方式,文件字串用三個引號引起來：

  def population_density(population, land_area):
    """Calculate the population density of an area. """
    return population / land_area

• 一行文件字串完全可接受, 可以在一行摘要後面新增更多資訊。我們還可以對函式的引數進行了解釋，描述每個引數的作用和型別。我們經常還會對函式輸出進行說明：

  def population_density(population, land_area):
    """Calculate the population density of an area.
  
    INPUT:
    population: int. The population of that area
    land_area: int or float. This function is unit-agnostic, if you pass in values in terms
    of square km or square miles the function will return a density in those units.
  
    OUTPUT: 
    population_density: population / land_area. The population density of a particular area.
    """
   
  
    return population / land_area