Question

我的问题是解释器运行相同的以下两段代码：

class A(object):
  def __init__(self):
     self.__x = None

  @property
  def x(self):
     if not self.__x:
        self.__x = ... #some complicated action
     return self.__x

更简单：

class A(object):
  @property
  def x(self):
      return ... #some complicated action

即，解释器是否足够智能以缓存属性x？

我的假设是x没有改变 - 发现它是硬，但一旦找到它就没有理由再找到它。

Answer 1

不，每次访问该属性时都会调用getter。

Answer 2

不需要添加memoize装饰器：

class memoized(object):
   """Decorator that caches a function's return value each time it is called.
   If called later with the same arguments, the cached value is returned, and
   not re-evaluated.
   """
   def __init__(self, func):
      self.func = func
      self.cache = {}
   def __call__(self, *args):
      try:
         return self.cache[args]
      except KeyError:
         value = self.func(*args)
         self.cache[args] = value
         return value
      except TypeError:
         # uncachable -- for instance, passing a list as an argument.
         # Better to not cache than to blow up entirely.
         return self.func(*args)
   def __repr__(self):
      """Return the function's docstring."""
      return self.func.__doc__
   def __get__(self, obj, objtype):
      """Support instance methods."""
      return functools.partial(self.__call__, obj)

@memoized
def fibonacci(n):
   "Return the nth fibonacci number."
   if n in (0, 1):
      return n
   return fibonacci(n-1) + fibonacci(n-2)

print fibonacci(12)

Answer 3

属性不会自动缓存其返回值。每次访问属性时都会调用getter（和setter）。

但是，Denis Otkidach编写了一个精彩的缓存属性装饰器（在the Python Cookbook, 2nd edition中发布，最初也在ActiveState下的PSF license上），仅用于此目的：

class cache(object):    
    '''Computes attribute value and caches it in the instance.
    Python Cookbook (Denis Otkidach) https://stackoverflow.com/users/168352/denis-otkidach
    This decorator allows you to create a property which can be computed once and
    accessed many times. Sort of like memoization.

    '''
    def __init__(self, method, name=None):
        # record the unbound-method and the name
        self.method = method
        self.name = name or method.__name__
        self.__doc__ = method.__doc__
    def __get__(self, inst, cls):
        # self: <__main__.cache object at 0xb781340c>
        # inst: <__main__.Foo object at 0xb781348c>
        # cls: <class '__main__.Foo'>       
        if inst is None:
            # instance attribute accessed on class, return self
            # You get here if you write `Foo.bar`
            return self
        # compute, cache and return the instance's attribute value
        result = self.method(inst)
        # setattr redefines the instance's attribute so this doesn't get called again
        setattr(inst, self.name, result)
        return result

以下是一个展示其用途的示例：

def demo_cache():
    class Foo(object):
        @cache
        def bar(self):
            print 'Calculating self.bar'  
            return 42
    foo=Foo()
    print(foo.bar)
    # Calculating self.bar
    # 42
    print(foo.bar)    
    # 42
    foo.bar=1
    print(foo.bar)
    # 1
    print(Foo.bar)
    # __get__ called with inst = None
    # <__main__.cache object at 0xb7709b4c>

    # Deleting `foo.bar` from `foo.__dict__` re-exposes the property defined in `Foo`.
    # Thus, calling `foo.bar` again recalculates the value again.
    del foo.bar
    print(foo.bar)
    # Calculating self.bar
    # 42

demo_cache()

Answer 4

Python 3.2 onwards提供了一个内置的装饰器，可用于创建LRU缓存：

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

或者，如果你正在使用Flask / Werkzeug，那就是@cached_property装饰者。

对于Django，请尝试from django.utils.functional import cached_property

Answer 5

对于任何可能在2020年阅读此书的人来说，此功能现已在funcutils模块中作为Python 3.8的标准库的一部分提供。

https://docs.python.org/dev/library/functools.html#functools.cached_property

重要的是，定义自己的__dict__（或根本不定义一个）或使用__slots__的类可能无法正常工作。例如NamedTuple和元类。

Answer 6

@ unutbu的答案中提到的Denis Otkidach的装饰者发表在O'Reilly的Python Cookbook中。不幸的是，O'Reilly没有为代码示例指定任何许可 - 就像重用代码的非正式许可一样。

如果您需要具有自由许可的缓存属性装饰器，则可以使用Ken Seehof中的ActiveState code recipes @cached_property。它明确发布在MIT license。

下

def cached_property(f):
    """returns a cached property that is calculated by function f"""
    def get(self):
        try:
            return self._property_cache[f]
        except AttributeError:
            self._property_cache = {}
            x = self._property_cache[f] = f(self)
            return x
        except KeyError:
            x = self._property_cache[f] = f(self)
            return x

    return property(get)

Answer 7

由于我有同样的问题，我不得不查一下。

标准库中的functools package也将获得cached_property装饰器。不幸的是，它仅在Python 3.8中可用（截止本文发布之时，它是3.8a0）。等待的另一种方法是暂时使用this one as mentioned by 0xc0de）或Django的自定义变量，然后再切换：

from django.utils.functional import cached_property
# from functools import cached_property # Only 3.8+ :(

Answer 8

注意：为了使可用选项完整，进行添加。

否，property默认不被缓存。但是，有几种方法可以实现该功能，我想再添加一个：

https://github.com/pydanny/cached-property

python：属性字段是自动缓存的吗？

8 个答案: