Question

我正在尝试为memoize编写python decorator。我几乎没有问题。

@memoize如何翻译成memoize类的调用函数？
为什么 init 期望参数。
缓存存储在哪里？它与每个函数相关联，还是全局变量？即如果我使用@memoize，会有两个缓存对象多功能。

...

class memoize:
    def __init__(self):
        self.cache = {}

    def __call__(self, function):
        def wrapper(*args, **kwargs):
            key = str(function.__name__) + str(args) + str(kwargs)
            if key in cache:
                return cache[key]
            else:
                value = function(*args, **kwargs)
                cache[key] = value
                return value
        return wrapper

@memoize
def fib(n):
    if n in (0, 1):
        return 1
    else:
        return fib(n-1) + fib(n-2)

for i in range(0, 10):
    print(fib(i))

我收到了编译错误。

Traceback (most recent call last):
  File "memoize.py", line 17, in <module>
    @memoize
TypeError: __init__() takes exactly 1 argument (2 given)

Answer 1

您应该记住，@decorator只是func = decorator(func)的语法糖。所以这里有所不同：

（1）

@decorator 
def func():
     ...

与

func = decorator(func)  # Just call of __init__
func(...)               # Call of decorator.__call__

但是（2）

@decorator(some_param)
def func():
     ...

与

# Call of __init__ plus call of __call__
func = decorator(some_param)(func)  
# Call of closure returned by decorator.__call__
func(...)

您已经为（2）语法实现了装饰器接受参数，但在使用它们时不提供它们，如示例（1）所示。这就是__init__抱怨的原因，它接收func作为第二个参数。
您应该在self.cache闭包中写wrapper，因此wrapper会引用相应的decorator对象。仅写cache将导致全局变量搜索，因此将失败。

UPD ：我将您的代码更改为接近（1）：

class memoize:
    def __init__(self, function):
        self.cache = {}
        self.function = function

    def __call__(self, *args, **kwargs):        
        key = str(args) + str(kwargs)
        if key in self.cache:
            return self.cache[key]

        value = self.function(*args, **kwargs)
        self.cache[key] = value
        return value

@memoize
def fib(n):
    if n in (0, 1):
        return 1
    else:
        return fib(n-1) + fib(n-2)

for i in range(0, 10):
    print(fib(i))

print(fib.cache)

Answer 2

This is a custom cache我已经满足了我的特定需求，并提供了一些额外的功能，例如清除缓存和更新密钥等等。

import datetime


class CustomCache:
    def __init__(self, function, max_size=100):
        """Constructor"""
        self.function = function
        self.cache = {}
        self.max_cache_size = max_size

    def __call__(self, *args, **kwargs):
        key = str(args) + str(kwargs)
        if key in self.cache:
            return self.cache[key]['value']

        value = self.function(*args, **kwargs)
        self.update(key, value)
        return value

    def __contains__(self, key):
        """
        Returns True or False depending on whether or not the key is in the cache
        """
        return key in self.cache

    def update(self, key, value):
        """
        Update the cache dictionary and optionally remove the oldest item
        """

        if key not in self.cache and len(self.cache) >= self.max_cache_size:
            self.remove_oldest()
        self.cache[key] = {'date_accessed': datetime.datetime.now(), 'value': value}

    def update_key(self, value, *args, **kwargs):
        """
        Update specific key in cache just in case
        """
        key = str(args) + str(kwargs)
        if key in self.cache:
            self.cache[key] = {'date_accessed': datetime.datetime.now(), 'value': value}

    def remove_oldest(self):
        """
        Remove the entry that has the oldest accessed date
        """

        oldest_entry = None
        for key in self.cache:
            if oldest_entry is None:
                oldest_entry = key
            elif self.cache[key]['date_accessed'] < self.cache[oldest_entry]['date_accessed']:
                oldest_entry = key
        self.cache.pop(oldest_entry)

    def cache_clear(self):
        """
        Clear entire cache 
        """
        self.cache = {}

    @property
    def size(self):
        """
        Return the size of the cache
        """

        return len(self.cache)

我怎么能写python装饰器进行缓存？

2 个答案: