項目中遇到的裝飾器

1. functools.partial

django 用法：

• 代碼出現的地方

  path = partial(_path, Pattern=RoutePattern)
  re_path = partial(_path, Pattern=RegexPattern)

• 詳解

  # 調用path時至關於調用_path('login/', LoginView.as_view(), Pattern=RoutePattern)
  urlpatterns = [
      path('login/', LoginView.as_view()),
  ]

2. functools.lru_cache

django:

• manage.py 命令行執行時fetch_command函數調用get_commands

• get_commands每次調用都會返回一個dict，當settings文件沒有改變的時，返回的值是不變的，使用裝飾器則減小了每次啓動服務計算commands的時間

  @functools.lru_cache(maxsize=None)
  def get_commands():
      commands = {name: 'django.core' for name in find_commands(__path__[0])}
  
      if not settings.configured:
          return commands
  
      for app_config in reversed(list(apps.get_app_configs())):
          path = os.path.join(app_config.path, 'management')
          commands.update({name: app_config.name for name in find_commands(path)})
  
      return commands

• functools.lru_cache(maxsize=128, typed=False)

  -- maxsize表明緩存的內存佔用值，超過這個值以後，以前的結果就會被釋放，而後將新的計算結果進行緩存,其值應當設爲2的冪
  -- typed若爲True，則會把不一樣的參數類型獲得的結果分開保存

• 做用：主要是用來作臨時緩存，把耗時的操做結果暫時保存，避免重複計算，好比生成斐波那契數列時，函數後覺得結果依賴前兩位計算結果值

3. classonlymethod 和 classmethod

• django CBV 中 類繼承View類，urlpattern裏面調用as_view實現一個接口不一樣不一樣調用方式走不一樣的邏輯，as_view方法使用@classonlymethod裝飾器
• 源碼

  class classonlymethod(classmethod):
       def __get__(self, instance, cls=None):
           if instance is not None:
               raise AttributeError("This method is available only on the class, not on instances.")
           return super().__get__(instance, cls)

• 源碼能夠看出classonlymethod和classmethod的區別即爲，classonlymethod只能由類調用，實例對象調用時會拋出對象異常

4. @functools.wraps(func) 用於定義裝飾器的時候，特別是多個函數被裝飾器裝飾時，保留原函數的名稱和屬性

• 使用示例：

  def my_decorator(func):      
       @functools.wraps(func)
       def wrapper(*args, **kwargs):
           '''do something'''
           return func(*args, **kwargs)
       return wrapper

• 源碼，根據源碼能夠看出wraps 函數實現了更新包裝器函數，將被包裝的函數屬性賦給新的包裝器函數並返回，因此說該函數的做用是保留被裝飾對象的屬性

  # 使用偏函數，對傳進來的包裝器函數調用update_wrapper
   # 返回一個裝飾器，該裝飾器使用裝飾後的函數做爲包裝器參數並調用wraps（）的參數做爲其他參數來調用update_wrapper（）。 
   # 默認參數與update_wrapper（）相同。
   def wraps(wrapped,
             assigned = WRAPPER_ASSIGNMENTS,
             updated = WRAPPER_UPDATES):
       """Decorator factory to apply update_wrapper() to a wrapper function
          Returns a decorator that invokes update_wrapper() with the decorated function as the wrapper argument and the arguments to wraps() as the remaining arguments. Default arguments are as for update_wrapper().
          This is a convenience function to simplify applying partial() to update_wrapper().
       """
       return partial(update_wrapper, wrapped=wrapped,
                      assigned=assigned, updated=updated)
   def update_wrapper(wrapper,
                      wrapped,
                      assigned = WRAPPER_ASSIGNMENTS,
                      updated = WRAPPER_UPDATES):
       """Update a wrapper function to look like the wrapped function wrapper is the function to be updated wrapped is the original function assigned is a tuple naming the attributes assigned directly from the wrapped function to the wrapper function (defaults to functools.WRAPPER_ASSIGNMENTS) updated is a tuple naming the attributes of the wrapper that are updated with the corresponding attribute from the wrapped function (defaults to functools.WRAPPER_UPDATES)
       """
       for attr in assigned:
           try:
               value = getattr(wrapped, attr)
           except AttributeError:
               pass
           else:
               setattr(wrapper, attr, value)
       for attr in updated:
           getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
       # Issue #17482: set __wrapped__ last so we don't inadvertently copy it      
       # from the wrapped function when updating __dict__
       wrapper.__wrapped__ = wrapped
       # Return the wrapper so this can be used as a decorator via partial()
       return wrapper