裝飾器
在stackoverflow上看到一篇講python中decorator的回答,實在是受益匪淺,決定将其翻譯成中文,分享給大家。
函數是對象
在python中,函數是對象。
以一個簡單的函數為例。def shout(word="yes"):
return word.capitalize()+"!"print(shout())
會輸出Yes!
作為對象,是可以指派給另外一個變量的。scream = shoutprint(scream())
輸出和上面是一樣的。
更有意思的是,python中的函數是可以定義在另外一個函數内部的。def talk():
def whisper(word="yes"):
return word.lower()+"..."
print(whisper())
talk()
輸出yes...
函數引用
通過上面的例子可以看到,函數是對象,是以可以被指派給另外一個變量
可以在另外一個函數中被定義。
這也就意味着,一個函數可以将另外一個函數作為傳回值。def getTalk(kind="shout"):
def shout(word="yes"):
return word.capitalize()+"!"
def whisper(word="yes"):
return word.lower()+"..."
if kind == "shout": return shout else: return whisper
talk = getTalk()print talk # 輸出 print talk() # 輸出 Yes!print getTalk("whisper")() # 輸出yes...
如果能夠傳回一個函數,也可以将一個函數作為參數傳入,複用上面的scream函數def doSomethingBefore(func):
print("I do something before then I call the function you gave me")
print(func)
doSomethingBefore(scream)
輸出I do something before then I call the function you gave me
Yes!
現在應該能夠了解裝飾器了吧,裝飾器實際上就是對函數進行了包裝,它能夠在不改變函數的前提下,在這個函數被執行之前或者執行之後執行一段代碼。
手寫裝飾器# 裝飾器将一個另外的函數作為參數傳入def my_shiny_new_decorator(a_function_to_decorate):
# 定義一個wrapper
def the_wrapper_around_the_original_function():
# 在傳入的函數被執行前執行
print("Before the function runs")
a_function_to_decorate() # 在傳入的函數被執行後執行
print("After the function runs") return the_wrapper_around_the_original_functiondef a_stand_alone_function():
print("I am a stand alone function, don't you dare modify me")
a_stand_alone_function()# 輸出: I am a stand alone function, don't you dare modify mea_stand_alone_function_decorated = my_shiny_new_decorator(a_stand_alone_function)
a_stand_alone_function_decorated()# 輸出 # Before the function runs# I am a stand alone function, don't you dare modify me# After the function runs
也許你想要每次調用a_stand_alone_function時,
a_stand_alone_function_decorated就會被調用,這很簡單,隻需要用a_stand_alone_function_decorated傳回的函數覆寫之前的a_stand_alone_function就可以了。
使用裝飾器@my_shiny_new_decoratordef another_stand_alone_function():
print("Leave me alone")
another_stand_alone_function()
# 輸出#Before the function runs#Leave me alone#After the function runs
是的,就是這麼簡單。 @decorator就是下面代碼的簡寫another_stand_alone_function = my_shiny_new_decorator(another_stand_alone_function)
像下面的代碼def bread(func):
def wrapper():
print("''''''\>")
func()
print("") return wrapperdef ingredients(func):
def wrapper():
print("#tomatoes#")
func()
print("~salad~") return wrapperdef sandwich(food="--ham--"):
print(food)
sandwich()#outputs: --ham--sandwich = bread(ingredients(sandwich))
sandwich()
就可以寫成@[email protected] sandwich(food="--ham--"):
print(food)
sandwich()#outputs:#''''''\># #tomatoes## --ham--# ~salad~#
裝飾器的順序很重要,如果改變上面的順序,函數的行為就被改變了@[email protected] sandwich(food="--ham--"):
print(food)
strange_sandwich()#outputs:##tomatoes##''''''\># --ham--## ~salad~
還是一個裝飾器的例子# The decorator to make it bolddef makebold(fn):
# The new function the decorator returns
def wrapper():
# Insertion of some code before and after
return "
" + fn() + ""
return wrapper# The decorator to make it italicdef makeitalic(fn):
# The new function the decorator returns
def wrapper():
# Insertion of some code before and after
return "" + fn() + ""
return [email protected]@makeitalicdef say():
return "hello"print(say())#outputs:
hello# 等同于下面的函數 def say():
return "hello"say = makebold(makeitalic(say))
print(say())#outputs:
hello裝飾器更進階的應用
向被裝飾的函數傳遞參數def a_decorator_passing_arguments(function_to_decorate):
def a_wrapper_accepting_arguments(arg1, arg2):
print("I got args! Look: {0}, {1}".format(arg1, arg2))
function_to_decorate(arg1, arg2) return a_wrapper_accepting_arguments# 因為最終被調用是被裝飾器傳回的函數,即wrapper,# 是以将參數傳遞給wrapper會将這些參數傳遞給被裝飾的函數@a_decorator_passing_argumentsdef print_full_name(first_name, last_name):
print("My name is {0} {1}".format(first_name, last_name))
print_full_name("Peter", "Venkman")# outputs:#I got args! Look: Peter Venkman#My name is Peter Venkman
裝飾方法
python中方法和函數是一樣的。唯一不同的是,方法期望傳入的第一個參數是目前的對象(self)。
也就是說裝飾方法和裝飾函數沒有差異,隻需要在裝飾方法的時候将第一個參數考慮進去就行def method_friendly_decorator(method_to_decorate): def wrapper(self, lie):
lie = lie - 3
return method_to_decorate(self, lie) return wrapperclass Lucy(object):
def __init__(self): self.age = 32
@method_friendly_decorator def sayYourAge(self, lie):
print("I am {0}, what did you think?".format(self.age + lie))
l = Lucy()
l.sayYourAge(-3)#outputs: I am 26, what did you think?
如果試圖些一個通用的裝飾器,可以用*args, **kwargsdef a_decorator_passing_arbitrary_arguments(function_to_decorate):
# The wrapper accepts any arguments
def a_wrapper_accepting_arbitrary_arguments(*args, **kwargs):
print("Do I have args?:")
print(args)
print(kwargs) # Then you unpack the arguments, here *args, **kwargs
# If you are not familiar with unpacking, check: # http://www.saltycrane.com/blog/2008/01/how-to-use-args-and-kwargs-in-python/
function_to_decorate(*args, **kwargs) return [email protected]_decorator_passing_arbitrary_argumentsdef function_with_no_argument():
print("Python is cool, no argument here.")
function_with_no_argument()#outputs#Do I have args?:#()#{}#Python is cool, no argument [email protected]_decorator_passing_arbitrary_argumentsdef function_with_arguments(a, b, c):
print(a, b, c)
function_with_arguments(1,2,3)#outputs#Do I have args?:#(1, 2, 3)#{}#1 2 3 @a_decorator_passing_arbitrary_argumentsdef function_with_named_arguments(a, b, c, platypus="Why not ?"):
print("Do {0}, {1} and {2} like platypus? {3}".format(a, b, c, platypus))
function_with_named_arguments("Bill", "Linus", "Steve", platypus="Indeed!")#outputs#Do I have args ? :#('Bill', 'Linus', 'Steve')#{'platypus': 'Indeed!'}#Do Bill, Linus and Steve like platypus? Indeed!class Mary(object):
def __init__(self):
self.age = 31 @a_decorator_passing_arbitrary_arguments
def sayYourAge(self, lie=-3):
# You can now add a default value
print("I am {0}, what did you think?".format(self.age + lie))
m = Mary()
m.sayYourAge()#outputs# Do I have args?:#(<__main__.mary>,)#{}#I am 28, what did you think?
向裝飾器傳遞參數
那麼問題來了,怎麼向裝飾器傳遞參數呢?
這有點讓人撓頭,因為裝飾器必須接受一個函數作為參數。
是以可以直接向裝飾器傳遞參數。
回想一下之前的代碼# Decorators are ORDINARY functionsdef my_decorator(func):
print("I am an ordinary function") def wrapper():
print("I am function returned by the decorator")
func() return wrapper# Therefore, you can call it without any "@"def lazy_function():
print("zzzzzzzz")
decorated_function = my_decorator(lazy_function)#outputs: I am an ordinary function# It outputs "I am an ordinary function", because that’s just what you do:# calling a function. Nothing [email protected]_decoratordef lazy_function():
print("zzzzzzzz")#outputs: I am an ordinary function
這兩個是一樣的。my_decorator被調用了。
當寫下@my_decorator時,python會調用被标注為my_decorator的函數def decorator_maker():
print("I make decorators! I am executed only once: "
"when you make me create a decorator.") def my_decorator(func):
print("I am a decorator! I am executed only when you decorate a function.") def wrapped():
print("I am the wrapper around the decorated function. " "I am called when you call the decorated function. "
"As the wrapper, I return the RESULT of the decorated function.") return func()
print("As the decorator, I return the wrapped function.") return wrapped
print("As a decorator maker, I return a decorator") return my_decorator# Let’s create a decorator. It’s just a new function after all.new_decorator = decorator_maker()
#outputs:#I make decorators! I am executed only once: when you make me create a decorator.#As a decorator maker, I return a decorator# Then we decorate the functiondef decorated_function():
print("I am the decorated function.")
decorated_function = new_decorator(decorated_function)#outputs:#I am a decorator! I am executed only when you decorate a function.#As the decorator, I return the wrapped function# Let’s call the function:decorated_function()#outputs:#I am the wrapper around the decorated function. I am called when you call the decorated function.#As the wrapper, I return the RESULT of the decorated function.#I am the decorated function.
将代碼中的中間變量去掉試試。def decorated_function():
print("I am the decorated function.")
decorated_function = decorator_maker()(decorated_function)#outputs:#I make decorators! I am executed only once: when you make me create a decorator.#As a decorator maker, I return a decorator#I am a decorator! I am executed only when you decorate a function.#As the decorator, I return the wrapped function.# Finally:decorated_function()
#outputs:#I am the wrapper around the decorated function. I am called when you call the decorated function.#As the wrapper, I return the RESULT of the decorated function.#I am the decorated function.
再簡化一次代碼@decorator_maker()def decorated_function():
print("I am the decorated function.")#outputs:#I make decorators! I am executed only once: when you make me create a decorator.#As a decorator maker, I return a decorator#I am a decorator! I am executed only when you decorate a function.#As the decorator, I return the wrapped function.#Eventually: decorated_function()
#outputs:#I am the wrapper around the decorated function. I am called when you call the decorated function.#As the wrapper, I return the RESULT of the decorated function.#I am the decorated function.
回到之前的問題,如果我們能夠随時生成裝飾器,我們也能向那個生成的裝飾器傳遞參數。def decorator_maker_with_arguments(decorator_arg1, decorator_arg2):
print("I make decorators! And I accept arguments: {0}, {1}".format(decorator_arg1, decorator_arg2)) def my_decorator(func):
# The ability to pass arguments here is a gift from closures.
# If you are not comfortable with closures, you can assume it’s ok,
# or read: http://stackoverflow.com/questions/13857/can-you-explain-closures-as-they-relate-to-python
print("I am the decorator. Somehow you passed me arguments: {0}, {1}".format(decorator_arg1, decorator_arg2)) # Don't confuse decorator arguments and function arguments!
def wrapped(function_arg1, function_arg2) :
print("I am the wrapper around the decorated function.\n"
"I can access all the variables\n"
"\t- from the decorator: {0} {1}\n"
"\t- from the function call: {2} {3}\n"
"Then I can pass them to the decorated function"
.format(decorator_arg1, decorator_arg2, function_arg1, function_arg2)) return func(function_arg1, function_arg2) return wrapped return [email protected]_maker_with_arguments("Leonard", "Sheldon")def decorated_function_with_arguments(function_arg1, function_arg2):
print("I am the decorated function and only knows about my arguments: {0}"
" {1}".format(function_arg1, function_arg2))
decorated_function_with_arguments("Rajesh", "Howard")#outputs:#I make decorators! And I accept arguments: Leonard Sheldon#I am the decorator. Somehow you passed me arguments: Leonard Sheldon#I am the wrapper around the decorated function. #I can access all the variables # - from the decorator: Leonard Sheldon # - from the function call: Rajesh Howard #Then I can pass them to the decorated function#I am the decorated function and only knows about my arguments: Rajesh Howard
下面這個就是帶參數的裝飾器c1 = "Penny"c2 = "Leslie"@decorator_maker_with_arguments("Leonard", c1)def decorated_function_with_arguments(function_arg1, function_arg2):
print("I am the decorated function and only knows about my arguments:"
" {0} {1}".format(function_arg1, function_arg2))
decorated_function_with_arguments(c2, "Howard")#outputs:#I make decorators! And I accept arguments: Leonard Penny#I am the decorator. Somehow you passed me arguments: Leonard Penny#I am the wrapper around the decorated function. #I can access all the variables # - from the decorator: Leonard Penny # - from the function call: Leslie Howard #Then I can pass them to the decorated function#I am the decorated function and only knows about my arguments: Leslie Howard
用上面的方法可以向裝飾器傳遞參數,也可以用 *args, **kwargs這樣的參數形式。
但是要記住,這樣的動态方法隻能被使用一次,就是導入這個腳本的時候,後面不能再動态使用。
裝飾一個裝飾器
看下面的裝飾器def decorator_with_args(decorator_to_enhance):
"""
This function is supposed to be used as a decorator.
It must decorate an other function, that is intended to be used as a decorator.
Take a cup of coffee.
It will allow any decorator to accept an arbitrary number of arguments,
saving you the headache to remember how to do that every time.
"""
# We use the same trick we did to pass arguments
def decorator_maker(*args, **kwargs):
# We create on the fly a decorator that accepts only a function
# but keeps the passed arguments from the maker.
def decorator_wrapper(func):
# We return the result of the original decorator, which, after all,
# IS JUST AN ORDINARY FUNCTION (which returns a function).
# Only pitfall: the decorator must have this specific signature or it won't work:
return decorator_to_enhance(func, *args, **kwargs) return decorator_wrapper return decorator_maker
他應該這樣使用# You create the function you will use as a decorator. And stick a decorator on it :-)# Don't forget, the signature is "decorator(func, *args, **kwargs)"@decorator_with_args def decorated_decorator(func, *args, **kwargs):
def wrapper(function_arg1, function_arg2):
print("Decorated with {0} {1}".format(args, kwargs)) return func(function_arg1, function_arg2) return wrapper# Then you decorate the functions you wish with your brand new decorated [email protected]_decorator(42, 404, 1024)def decorated_function(function_arg1, function_arg2):
print("Hello {0} {1}".format(function_arg1, function_arg2))
decorated_function("Universe and", "everything")#outputs:#Decorated with (42, 404, 1024) {}#Hello Universe and everything# Whoooot!
最佳實踐裝飾器在python2.4後被引入,使用的時候确認版本支援。
使用裝飾器會增加函數調用的時間。
被裝飾的函數不能取消裝飾
裝飾器使得代碼更加難以調試
注意,使用裝飾器會會有一些副作用,被裝飾的函數其實已經是另外一個函數了。
為了消除這個副作用,可以使用functools.wraps這個方法。看下面的例子。# For debugging, the stacktrace prints you the function __name__def foo():
print("foo")
print(foo.__name__)#outputs: foo# With a decorator, it gets messy def bar(func):
def wrapper():
print("bar") return func() return [email protected] foo():
print("foo")
print(foo.__name__)#outputs: wrapper# "functools" can help for thatimport functoolsdef bar(func):
# We say that "wrapper", is wrapping "func"
# and the magic begins @functools.wraps(func)
def wrapper():
print("bar") return func() return [email protected] foo():
print("foo")
print(foo.__name__)#outputs: foo
實際使用
裝飾器可以在很多場景下使用。
比如倒入某個外部庫的時候,可以使用裝飾器擴充庫中函數的行為。
或者在debug的時候使用。def benchmark(func):
"""
A decorator that prints the time a function takes to execute.
"""
import time def wrapper(*args, **kwargs):
t = time.clock()
res = func(*args, **kwargs)
print("{0} {1}".format(func.__name__, time.clock()-t)) return res return wrapperdef logging(func):
"""
A decorator that logs the activity of the script. (it actually just prints it, but it could be logging!)
"""
def wrapper(*args, **kwargs):
res = func(*args, **kwargs)
print("{0} {1} {2}".format(func.__name__, args, kwargs)) return res return wrapperdef counter(func):
"""
A decorator that counts and prints the number of times a function has been executed
"""
def wrapper(*args, **kwargs):
wrapper.count = wrapper.count + 1
res = func(*args, **kwargs)
print("{0} has been used: {1}x".format(func.__name__, wrapper.count)) return res
wrapper.count = 0
return [email protected]@[email protected] reverse_string(string):
return str(reversed(string))
print(reverse_string("Able was I ere I saw Elba"))
print(reverse_string("A man, a plan, a canoe, pasta, heros, rajahs, a coloratura, maps, snipe, percale, macaroni, a gag, a banana bag, a tan, a tag, a banana bag again (or a camel), a crepe, pins, Spam, a rut, a Rolo, cash, a jar, sore hats, a peon, a canal: Panama!"))#outputs:#reverse_string ('Able was I ere I saw Elba',) {}#wrapper 0.0#wrapper has been used: 1x#ablE was I ere I saw elbA#reverse_string ('A man, a plan, a canoe, pasta, heros, rajahs, a coloratura, maps, snipe, percale, macaroni, a gag, a banana bag, a tan, a tag, a banana bag again (or a camel), a crepe, pins, Spam, a rut, a Rolo, cash, a jar, sore hats, a peon, a canal: Panama!',) {}#wrapper 0.0#wrapper has been used: 2x#!amanaP :lanac a ,noep a ,stah eros ,raj a ,hsac ,oloR a ,tur a ,mapS ,snip ,eperc a ,)lemac a ro( niaga gab ananab a ,gat a ,nat a ,gab ananab a ,gag a ,inoracam ,elacrep ,epins ,spam ,arutaroloc a ,shajar ,soreh ,atsap ,eonac a ,nalp a ,nam A
使用裝飾器可以少寫很多重複的代碼@[email protected]@loggingdef get_random_futurama_quote():
from urllib import urlopen
result = urlopen("http://subfusion.net/cgi-bin/quote.pl?quote=futurama").read() try:
value = result.split("
")[1].split("")[0] return value.strip() except: return "No, I'm ... doesn't!"print(get_random_futurama_quote())print(get_random_futurama_quote())#outputs:#get_random_futurama_quote () {}#wrapper 0.02#wrapper has been used: 1x#The laws of science be a harsh mistress.#get_random_futurama_quote () {}#wrapper 0.01#wrapper has been used: 2x#Curse you, merciful Poseidon!
python提供了很多的内在裝飾器,如property,staticmethod等等。
還有一些其它的庫也用到了裝飾器Django用裝飾器來管理緩存和視圖函數的權限
Twisted用裝飾器來僞造内聯異步調用
作者:大蟒傳奇
連結:https://www.jianshu.com/p/954736037176