python裝飾器property和setter用法

更新時(shí)間：2022年07月15日 17:28:28 作者：wx62cea850b9e28???????

這篇文章主要介紹了python裝飾器property和setter用法，文章圍繞主題展開詳細(xì)的內(nèi)容介紹，具有一定的參考價(jià)值，需要的朋友可以參考一下

1.引子：函數(shù)也是對象

木有括號的函數(shù)那就不是在調(diào)用。

def hi(name="yasoob"):
return "hi " + name
print(hi())
# output: 'hi yasoob'
# 我們甚至可以將一個(gè)函數(shù)賦值給一個(gè)變量，比如
greet = hi
# 我們這里沒有在使用小括號，因?yàn)槲覀儾⒉皇窃谡{(diào)用hi函數(shù)
# 而是在將它放在greet變量里頭。我們嘗試運(yùn)行下這個(gè)
print(greet())
# output: 'hi yasoob'
# 如果我們刪掉舊的hi函數(shù)，看看會(huì)發(fā)生什么！
del hi
print(hi())
#outputs: NameError
print(greet())
#outputs: 'hi yasoob'

2.函數(shù)內(nèi)的函數(shù)

（1）在python中，一個(gè)函數(shù)內(nèi)能嵌套定義另一個(gè)函數(shù)，并且可以在該大函數(shù)內(nèi)調(diào)用該小函數(shù)。

def hi(name="yasoob"):
print("now you are inside the hi() function")

def greet():
return "now you are in the greet() function"

def welcome():
return "now you are in the welcome() function"

print(greet())
print(welcome())
print("now you are back in the hi() function")

hi()
#output:now you are inside the hi() function
# now you are in the greet() function
# now you are in the welcome() function
# now you are back in the hi() function

# 上面展示了無論何時(shí)你調(diào)用hi(), greet()和welcome()將會(huì)同時(shí)被調(diào)用。
# 然后greet()和welcome()函數(shù)在hi()函數(shù)之外是不能訪問的，比如：
greet()
#outputs: NameError: name 'greet' is not defined

（2）開始神奇的是，大函數(shù)的返回值可以是一個(gè)函數(shù)：

def hi(name="yasoob"):
def greet():
return "now you are in the greet() function"
def welcome():
return "now you are in the welcome() function"
if name == "yasoob":
return greet #這里?。?
else:
return welcome
a = hi()
print(a)
#outputs: <function greet at 0x7f2143c01500>
#上面清晰地展示了`a`現(xiàn)在指向到hi()函數(shù)中的greet()函數(shù)
#現(xiàn)在試試這個(gè)
print(a())
#outputs: now you are in the greet() function

在 if/else 語句中我們返回 greet 和 welcome，而不是 greet() 和 welcome()。
為什么那樣？這是因?yàn)楫?dāng)你把一對小括號放在后面，這個(gè)函數(shù)就會(huì)執(zhí)行；然而如果你不放括號在它后面，那它可以被到處傳遞，并且可以賦值給別的變量而不去執(zhí)行它。

當(dāng)我們寫下 a = hi()，hi() 會(huì)被執(zhí)行，而由于 name 參數(shù)默認(rèn)是 yasoob，所以函數(shù) greet 被返回了。

PS：如果我們打印出 hi()()，這會(huì)輸出 now you are in the greet() function。

（3）最后要說的是函數(shù)作為參數(shù)傳入一個(gè)函數(shù)：

def hi():
return "hi yasoob!"
def doSomethingBeforeHi(func):
print("I am doing some boring work before executing hi()")
print(func())
doSomethingBeforeHi(hi)
#outputs:I am doing some boring work before executing hi()
# hi yasoob!

3.裝飾器小栗子

終于來到了帶@的裝飾器，其實(shí)就是帶了@帽子的函數(shù)作為參數(shù)，傳入@后面的函數(shù)中。

def a_new_decorator(a_func):
def wrapTheFunction():
print("I am doing some boring work before executing a_func()")
a_func()
print("I am doing some boring work after executing a_func()")
return wrapTheFunction
@a_new_decorator
def a_function_requiring_decoration():
"""Hey you! Decorate me!"""
print("I am the function which needs some decoration to "
"remove my foul smell")

a_function_requiring_decoration()
#outputs: I am doing some boring work before executing a_func()
# I am the function which needs some decoration to remove my foul smell
# I am doing some boring work after executing a_func()
#the @a_new_decorator is just a short way of saying:
a_function_requiring_decoration = a_new_decorator(a_function_requiring_decoration)

上面的代碼等價(jià)于我們熟悉的：

def a_new_decorator(a_func):
def wrapTheFunction():
print("I am doing some boring work before executing a_func()")
a_func()
print("I am doing some boring work after executing a_func()")
return wrapTheFunction

def a_function_requiring_decoration():
print("I am the function which needs some decoration to remove my foul smell")

a_function_requiring_decoration()
#outputs: "I am the function which needs some decoration to remove my foul smell"
a_function_requiring_decoration = a_new_decorator(a_function_requiring_decoration)
#now a_function_requiring_decoration is wrapped by wrapTheFunction()
a_function_requiring_decoration()
#outputs:I am doing some boring work before executing a_func()
# I am the function which needs some decoration to remove my foul smell
# I am doing some boring work after executing a_func()

不過一開始上面被裝飾過的函數(shù)名字已經(jīng)悄悄發(fā)生“改變”，如果print下可以看出（如下代碼）。
解決方案：
@wraps接受一個(gè)函數(shù)來進(jìn)行裝飾，并加入了復(fù)制函數(shù)名稱、注釋文檔、參數(shù)列表等等的功能。這可以讓我們在裝飾器里面訪問在裝飾之前的函數(shù)的屬性。

print(a_function_requiring_decoration.__name__)
# Output: wrapTheFunction

最終加上@wraps的代碼如下：

from functools import wraps
def a_new_decorator(a_func):
@wraps(a_func)
def wrapTheFunction():
print("I am doing some boring work before executing a_func()")
a_func()
print("I am doing some boring work after executing a_func()")
return wrapTheFunction
@a_new_decorator
def a_function_requiring_decoration():
"""Hey yo! Decorate me!"""
print("I am the function which needs some decoration to "
"remove my foul smell")
print(a_function_requiring_decoration.__name__)
# Output: a_function_requiring_decoration

5.property和setter用法

class Timer:
def __init__(self, value = 0.0):
self._time = value
self._unit = 's'
# 使用裝飾器的時(shí)候，需要注意：
# 1. 裝飾器名，函數(shù)名需要一直
# 2. property需要先聲明，再寫setter，順序不能倒過來
@property
def time(self):
return str(self._time) + ' ' + self._unit
@time.setter
def time(self, value):
if(value < 0):
raise ValueError('Time cannot be negetive.')
self._time = value
t = Timer()
t.time = 1.0
print(t.time)

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