众所周知，下面两段代码几乎是等价的:
@dec
def foo():
pass foo = dec(foo)
############################################
foo = dec(foo)
一个常见的错误是认为@只是隐藏了最左边的参数。
@dec(1, 2, 3)
def foo():
pass
###########################################
foo = dec(foo, 1, 2, 3)
如果@是这样工作的，那么编写装饰器会容易得多。不幸的是，事情不是这样的。
考虑decorator Waitwhich haults
程序执行几秒钟。
如果你没有通过等待时间
缺省值为1秒。
用例如下所示。
##################################################
@Wait
def print_something(something):
print(something)
##################################################
@Wait(3)
def print_something_else(something_else):
print(something_else)
##################################################
@Wait(delay=3)
def print_something_else(something_else):
print(something_else)
当Wait有一个参数，比如@Wait(3)，那么调用Wait(3)
在发生任何其他事情之前执行。
也就是说，下面两段代码是等价的
@Wait(3)
def print_something_else(something_else):
print(something_else)
###############################################
return_value = Wait(3)
@return_value
def print_something_else(something_else):
print(something_else)
这是一个问题。
if `Wait` has no arguments:
`Wait` is the decorator.
else: # `Wait` receives arguments
`Wait` is not the decorator itself.
Instead, `Wait` ***returns*** the decorator
解决方案如下:
让我们从创建以下类开始，DelayedDecorator:
class DelayedDecorator:
def __init__(i, cls, *args, **kwargs):
print("Delayed Decorator __init__", cls, args, kwargs)
i._cls = cls
i._args = args
i._kwargs = kwargs
def __call__(i, func):
print("Delayed Decorator __call__", func)
if not (callable(func)):
import io
with io.StringIO() as ss:
print(
"If only one input, input must be callable",
"Instead, received:",
repr(func),
sep="\n",
file=ss
)
msg = ss.getvalue()
raise TypeError(msg)
return i._cls(func, *i._args, **i._kwargs)
现在我们可以这样写:
dec = DelayedDecorator(Wait, delay=4)
@dec
def delayed_print(something):
print(something)
注意:
dec does not not accept multiple arguments.
dec only accepts the function to be wrapped.
import inspect
class PolyArgDecoratorMeta(type):
def call(Wait, *args, **kwargs):
try:
arg_count = len(args)
if (arg_count == 1):
if callable(args[0]):
SuperClass = inspect.getmro(PolyArgDecoratorMeta)[1]
r = SuperClass.call(Wait, args[0])
else:
r = DelayedDecorator(Wait, *args, **kwargs)
else:
r = DelayedDecorator(Wait, *args, **kwargs)
finally:
pass
return r
import time
class Wait(metaclass=PolyArgDecoratorMeta):
def init(i, func, delay = 2):
i._func = func
i._delay = delay
def __call__(i, *args, **kwargs):
time.sleep(i._delay)
r = i._func(*args, **kwargs)
return r
下面两段代码是等价的:
@Wait
def print_something(something):
print (something)
##################################################
def print_something(something):
print(something)
print_something = Wait(print_something)
我们可以非常缓慢地将“something”打印到控制台，如下所示:
print_something("something")
#################################################
@Wait(delay=1)
def print_something_else(something_else):
print(something_else)
##################################################
def print_something_else(something_else):
print(something_else)
dd = DelayedDecorator(Wait, delay=1)
print_something_else = dd(print_something_else)
##################################################
print_something_else("something")
最后指出
它可能看起来有很多代码，但你不必每次都写类DelayedDecorator和PolyArgDecoratorMeta。你必须亲自编写的代码如下所示，这是相当短的:
from PolyArgDecoratorMeta import PolyArgDecoratorMeta
import time
class Wait(metaclass=PolyArgDecoratorMeta):
def __init__(i, func, delay = 2):
i._func = func
i._delay = delay
def __call__(i, *args, **kwargs):
time.sleep(i._delay)
r = i._func(*args, **kwargs)
return r
