1、寫一個裝飾器，實現緩存功能，允許過期，但沒有換出，沒有清除
1）cache的必要元素：key --> value
這裏的key是函數的參數，value是函數的返回值
2）超時時間
超時時間如何存儲
步驟1：
In [28]: from functools import wraps
In [29]: def cache(fn):
...: cache_dict = {}
...: @wraps
...: def wrap(*args, **kwargs):
...: # 如何拼裝key
...: if key in cache_dict.keys():
...: # 如何實現超時檢測
...: return cache_dict[key]
...: result = fn(*args, **kwargs)
...: cache_dict[key] = result
...: return result
...: reutrn wrap如何拼裝key？
參數名 + 參數值
這裏需要用到inspect庫
標準庫inspect的用法：
In [1]: import inspect
In [2]: def add(x, y):
...: return x + Y
...:
In [3]: inspect.signature
Out[3]: <function inspect.signature>
In [4]: inspect.signature(add)
Out[4]: <Signature (x, y)>
In [5]: sig = inspect.signature(add) # 獲取到函數的簽名
In [17]: sig.parameters
Out[17]: mappingproxy({'x': <Parameter "x">, 'y': <Parameter "y">})
In [18]: for k in sig.parameters.keys(): # 獲取參數
...: print(k)
...:
x
y步驟2：
拼接key
In [30]: from functools import wraps
In [31]: import inspect
In [32]: def cache(fn):
...: cache_dict = {}
...: @wraps
...: def wrap(*args, **kwargs):
...: key = []
...: params = inspect.signature(fn).parameters
...: for i, arg in enumerate(args):
...: # 位置參數的名（行參x或y）和其值（實參）
...: name = list(params.keys())[i]
...: key.append((name, arg))
...: key.extend(kwargs.items())
...: # 拼接KEY
...: key.sort(key=lambda x: x[0]
...: key = '&'.join(['{}={}'.format(name, arg) for name, arg in key])
...:
...: if key in cache_dict.keys():
...: # 如何實現超時檢測
...: return cache_dict[key]
...: result = fn(*args, **kwargs)
...: cache_dict[key] = result
...: return result
...: reutrn wrap步驟3：
當被裝飾的函數有默認參數時，造成key不一樣，不走cache,怎麼處理？
## inspect標準庫的應用
# 當被裝飾的參數有默認參數時
In [14]: def add(x, y=2):
...: return x + y
...:
In [15]: sig = inspect.signature(add)
In [16]: sig
Out[16]: <Signature (x, y=2)>
In [19]: params = sig.parameters
In [20]: params
Out[20]: mappingproxy({'x': <Parameter "x">, 'y': <Parameter "y=2">})
In [24]: for k,v in params.items():
...: print(k, v)
...:
...:
x x
y y=2
In [25]: for k,v in params.items():
...: print(k, v.default)
...:
...:
...:
x <class 'inspect._empty'> # v.default
y 2處理默認參數：
In [48]: def cache(fn):
...: cache_dict = {}
...: @wraps(fn)
...: def wrap(*args, **kwargs):
...: key = []
...: names = set()
...: params = inspect.signature(fn).parameters
...: for i, arg in enumerate(args):
...: #
...: name = list(params.keys())[i]
...: key.append((name, arg))
...: names.add(name)
...: key.extend(kwargs.items())
...: names.update(kwargs.keys())
...: for k, v in params.items():
...: if k not in names:
...: key.append((k, v.default))
...: # 拼接KEY
...: key.sort(key=lambda x: x[0])
...: key = '&'.join(['{}={}'.format(name, arg) for name, arg in key])
...: print(key)
...: if key in cache_dict.keys():
...: # 如何實現超時檢測
...: return cache_dict[key]
...: result = fn(*args, **kwargs)
...: cache_dict[key] = result
...: return result
...: return wrap
...:
In [49]: @cache
...: def add(x, y=3):
...: return x + y
...:
In [50]: add(1, 3)
x=1&y=3
Out[50]: 4
In [51]: add(1)
x=1&y=3
Out[51]: 4
In [52]: add(x=1, y=3)
x=1&y=3
Out[52]: 4
In [53]: add(y=3, x=1)
x=1&y=3
Out[53]: 4步驟4：
實現過期功能
應該保存第一次傳入該key和生成其值的時間
過期時間也應該由用戶傳入，那應該在哪裏傳入呢？
可以使用在裝飾參數傳入
如果在被裝飾的參數傳入，就比較麻煩了
In [61]: def cache(exp=0):
...: def _cache(fn):
...: cache_dict = {}
...: @wraps(fn)
...: def wrap(*args, **kwargs):
...: key = []
...: names = set()
...: params = inspect.signature(fn).parameters
...: for i, arg in enumerate(args):
...: #
...: name = list(params.keys())[i]
...: key.append((name, arg))
...: names.add(name)
...: key.extend(kwargs.items())
...: names.update(kwargs.keys())
...: for k, v in params.items():
...: if k not in names:
...: key.append((k, v.default))
...: # 拼接KEY
...: key.sort(key=lambda x: x[0])
...: key = '&'.join(['{}={}'.format(name, arg) for name, arg in key])
...: print(key)
...: if key in cache_dict.keys():
...: result, timestamp = cache_dict[key]
...: if exp == 0 or datetime.datetime.now().timestamp() - timestamp < exp:
...: print("cache hit")
...: return cache_dict[key]
...: result = fn(*args, **kwargs)
...: cache_dict[key] = (result, datetime.datetime.now().timestamp())
...: print("cache miss")
...: return result
...: return wrap
...: return _cache
...:
In [68]: add(2, 3) # 第一次執行沒有命中
x=2&y=3
cache miss
Out[68]: 5
In [69]: add(2) # 5s內再次執行命中
x=2&y=3
cache hit
Out[69]: (5, 1497970027.772658)
In [70]: add(2, 3) # 過5s後再執行沒有命中
x=2&y=3
cache miss
Out[70]: 5可能會有人擔心沒有區分函數，在裝飾不同的函數，參數一樣時，會混：
In [71]: @cache(500) ...: def add(x, y=3): ...: return x + y ...: In [72]: add(1, 3) x=1&y=3 cache miss Out[72]: 4 In [73]: add(1, 3) x=1&y=3 cache hit Out[73]: (4, 1497970231.435554) In [74]: add(1, 3) x=1&y=3 cache hit Out[74]: (4, 1497970231.435554) In [76]: @cache(500) ...: def xxj(x, y=3): ...: return x + y ...: In [77]: xxj(1) x=1&y=3 cache miss Out[77]: 4
並沒有混，爲什麼？
2、裝飾器的用途
AOP：面向方面
針對一類問題做處理，與具體實現無關
裝飾器常見的使用場景：
監控、緩存、路由、權限、參數檢查