python裝飾器property和setter用法

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1.引子：函數也是對象

木有括號的函數那就不是在調用。

def hi(name="yasoob"):
return "hi " + name
print(hi())
# output: 'hi yasoob'
# 我們甚至可以將一個函數賦值給一個變量，比如
greet = hi
# 我們這裡沒有在使用小括號，因為我們並不是在調用hi函數
# 而是在將它放在greet變量裡頭。我們嘗試運行下這個
print(greet())
# output: 'hi yasoob'
# 如果我們刪掉舊的hi函數，看看會發生什麼！
del hi
print(hi())
#outputs: NameError
print(greet())
#outputs: 'hi yasoob'

2.函數內的函數

（1）在python中，一個函數內能嵌套定義另一個函數，並且可以在該大函數內調用該小函數。

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

# 上面展示瞭無論何時你調用hi(), greet()和welcome()將會同時被調用。
# 然後greet()和welcome()函數在hi()函數之外是不能訪問的，比如：
greet()
#outputs: NameError: name 'greet' is not defined

（2）開始神奇的是，大函數的返回值可以是一個函數：

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 #這裡！！
else:
return welcome
a = hi()
print(a)
#outputs: <function greet at 0x7f2143c01500>
#上面清晰地展示瞭`a`現在指向到hi()函數中的greet()函數
#現在試試這個
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()。
為什麼那樣？這是因為當你把一對小括號放在後面，這個函數就會執行；然而如果你不放括號在它後面，那它可以被到處傳遞，並且可以賦值給別的變量而不去執行它。

當我們寫下 a = hi()，hi() 會被執行，而由於 name 參數默認是 yasoob，所以函數 greet 被返回瞭。

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

（3）最後要說的是函數作為參數傳入一個函數：

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

終於來到瞭帶@的裝飾器，其實就是帶瞭@帽子的函數作為參數，傳入@後面的函數中。

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)

上面的代碼等價於我們熟悉的：

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名字已經悄悄發生“改變”，如果print下可以看出（如下代碼）。
解決方案：
@wraps接受一個函數來進行裝飾，並加入瞭復制函數名稱、註釋文檔、參數列表等等的功能。這可以讓我們在裝飾器裡面訪問在裝飾之前的函數的屬性。

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'
# 使用裝飾器的時候，需要註意：
# 1. 裝飾器名，函數名需要一直
# 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)

到此這篇關於python裝飾器property和setter用法的文章就介紹到這瞭,更多相關python property與setter內容請搜索WalkonNet以前的文章或繼續瀏覽下面的相關文章希望大傢以後多多支持WalkonNet！

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