Python decorator module
Python decorator module document
使用functool中的wraps,可以复制一些信息(__name__, __doc__, __module__, __dict__),但是signature还是会被改变(比如参数信息),要保留正确的参数信息,可以使用decorate,从接口上看decorate和update_wrapper相反,这和api的命名就有关了:
update_wrapper wrapper with wrapped.
decorate function with a caller function (the caller function describing the functionality of the decorator).
特别之处在于,decorate方法要求参数中的caller function具备完整的signature
The caller function must have signature (f, *args, **kw), and it must call the original function f with arguments args and kw, implementing the wanted capability (in this case, memoization)
以下面的例子来说,_memoize是caller function,memoize是decorator
def _memoize(func, *args, **kw): if kw: # frozenset is used to ensure hashability key = args, frozenset(kw.items()) else: key = args cache = func.cache # attribute added by memoize if key not in cache: cache[key] = func(*args, **kw) return cache[key] def memoize(f): """ A simple memoize implementation. It works by adding a .cache dictionary to the decorated function. The cache will grow indefinitely, so it is your responsability to clear it, if needed. """ f.cache = {} return decorate(f, _memoize) >>> @memoize ... def heavy_computation(): ... time.sleep(2) ... return "done" >>> print(getargspec(heavy_computation)) ArgSpec(args=[], varargs=None, varkw=None, defaults=None)
使用decorator模块可以防止更改signature,这样decorator符合一个signature-preserving decorators的要求:
Callable objects which accept a function as input and return a function as output, with the same signature.
来看另外一个例子
def _trace(f, *args, **kw): kwstr = ', '.join('%r: %r' % (k, kw[k]) for k in sorted(kw)) print("calling %s with args %s, {%s}" % (f.__name__, args, kwstr)) return f(*args, **kw) def trace(f): return decorate(f, _trace)
先了解一下结构:
python中单下划线开头代表这是一个内部函数,这里是_trace;
_trace是decorator内部描述装饰器功能的一个函数,也可以说是wrapper,f是那个wrapped;
与之前的memoize不同,这里的trace只返回内部函数_trace,不声明其余内容;
所以可以加语法糖!使用@decorator就达到一样的效果,将一个caller function转换成一个signature-reserving decorator。
>>> @decorator ... def trace(f, *args, **kw): ... kwstr = ', '.join('%r: %r' % (k, kw[k]) for k in sorted(kw)) ... print("calling %s with args %s, {%s}" % (f.__name__, args, kwstr)) ... return f(*args, **kw)
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