1  import types
       2  import functools
       3  
       4  
       5  # from jaraco.functools 3.3
       6  def method_cache(method, cache_wrapper=None):
       7      """
       8      Wrap lru_cache to support storing the cache data in the object instances.
       9  
      10      Abstracts the common paradigm where the method explicitly saves an
      11      underscore-prefixed protected property on first call and returns that
      12      subsequently.
      13  
      14      >>> class MyClass:
      15      ...     calls = 0
      16      ...
      17      ...     @method_cache
      18      ...     def method(self, value):
      19      ...         self.calls += 1
      20      ...         return value
      21  
      22      >>> a = MyClass()
      23      >>> a.method(3)
      24      3
      25      >>> for x in range(75):
      26      ...     res = a.method(x)
      27      >>> a.calls
      28      75
      29  
      30      Note that the apparent behavior will be exactly like that of lru_cache
      31      except that the cache is stored on each instance, so values in one
      32      instance will not flush values from another, and when an instance is
      33      deleted, so are the cached values for that instance.
      34  
      35      >>> b = MyClass()
      36      >>> for x in range(35):
      37      ...     res = b.method(x)
      38      >>> b.calls
      39      35
      40      >>> a.method(0)
      41      0
      42      >>> a.calls
      43      75
      44  
      45      Note that if method had been decorated with ``functools.lru_cache()``,
      46      a.calls would have been 76 (due to the cached value of 0 having been
      47      flushed by the 'b' instance).
      48  
      49      Clear the cache with ``.cache_clear()``
      50  
      51      >>> a.method.cache_clear()
      52  
      53      Same for a method that hasn't yet been called.
      54  
      55      >>> c = MyClass()
      56      >>> c.method.cache_clear()
      57  
      58      Another cache wrapper may be supplied:
      59  
      60      >>> cache = functools.lru_cache(maxsize=2)
      61      >>> MyClass.method2 = method_cache(lambda self: 3, cache_wrapper=cache)
      62      >>> a = MyClass()
      63      >>> a.method2()
      64      3
      65  
      66      Caution - do not subsequently wrap the method with another decorator, such
      67      as ``@property``, which changes the semantics of the function.
      68  
      69      See also
      70      http://code.activestate.com/recipes/577452-a-memoize-decorator-for-instance-methods/
      71      for another implementation and additional justification.
      72      """
      73      cache_wrapper = cache_wrapper or functools.lru_cache()
      74  
      75      def wrapper(self, *args, **kwargs):
      76          # it's the first call, replace the method with a cached, bound method
      77          bound_method = types.MethodType(method, self)
      78          cached_method = cache_wrapper(bound_method)
      79          setattr(self, method.__name__, cached_method)
      80          return cached_method(*args, **kwargs)
      81  
      82      # Support cache clear even before cache has been created.
      83      wrapper.cache_clear = lambda: None
      84  
      85      return wrapper
      86  
      87  
      88  # From jaraco.functools 3.3
      89  def pass_none(func):
      90      """
      91      Wrap func so it's not called if its first param is None
      92  
      93      >>> print_text = pass_none(print)
      94      >>> print_text('text')
      95      text
      96      >>> print_text(None)
      97      """
      98  
      99      @functools.wraps(func)
     100      def wrapper(param, *args, **kwargs):
     101          if param is not None:
     102              return func(param, *args, **kwargs)
     103  
     104      return wrapper