python中a=a+2與a+=2的區別

1、a=a+2，表示一個新的對象，新的對象名字還是a，但是指向的內存地址已經變了

>>> a=2
>>> id(a)
140406287260016
>>> a=a+2
>>> a
4
>>> id(a)
140406287259968

所以對於tuple對象（不可變對象），也是可以這樣操作的

>>> tuple1=(1,2)
>>> id(tuple1)
4521580448
>>> tuple1=tuple1+(3,)
>>> tuple1
(1, 2, 3)
>>> id(tuple1)
4521658880

2、a+=2對於有些對象的操作是表示原來的對象，對有些對象的操作是生成了一個新對象

不可變對象tuple1，操作完后，內存地址已經發生變化，生成一個新的對象
>>> tuple1=(1,2)
>>> type(tuple1)
<type 'tuple'>
>>> tuple1+=(3,)
>>> id(tuple1)
4521658880
>>> tuple1+=(4,5)
>>> id(tuple1)
4520649072

而list對象，可變對象，+=操作、append操作、extend操作，都是在原對象上操作

>>> list1=[1,2]
>>> id(list1)
4521614656
>>> list1+=[3]
>>> id(list1)
4521614656
>>> list1.append(4)
>>> id(list1)
4521614656
>>> list1.extend(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
>>> list1.extend([5])
>>> id(list1)
4521614656
>>>

3、

x = [1,2,3]  
print "before func(), global! x = ",x,"id(x) = ",id(x)  
  
def func():  
    global x  
    print "in func(), local! original x = ",x,"id(x) = ",id(x)  
    x = x + [1]  
    print "in func(), local! now x = ",x,"id(x) = ",id(x)  
  
func()  
print "after func(), global! x = ",x,"id(x) = ",id(x)  
結果：
[python] view plain copy
before func(), global! x =  [1, 2, 3] id(x) =  47781768  
in func(), local! original x =  [1, 2, 3] id(x) =  47781768  
in func(), local! now x =  [1, 2, 3, 1] id(x) =  47795720  
after func(), global! x =  [1, 2, 3, 1] id(x) =  47795720  

global就保證了，即使我的變量x在函數中指向對象變了，外部的x也會指向新的對象

 

x = [1,2,3]  
print "before func(), global! x = ",x,"id(x) = ",id(x)  
  
def func(x):  
    print "in func(), local! original x = ",x,"id(x) = ",id(x)  
    x  = x + [1]  
    print "in func(), local! now x = ",x,"id(x) = ",id(x)  
  
func(x)  
print "after func(), global! x = ",x,"id(x) = ",id(x)  
結果：
before func(), global! x =  [1, 2, 3] id(x) =  46339976  
in func(), local! original x =  [1, 2, 3] id(x) =  46339976  
in func(), local! now x =  [1, 2, 3, 1] id(x) =  46390664  
after func(), global! x =  [1, 2, 3] id(x) =  46339976  

x = x + [1]，是新建了一個對象，id(x) =  46390664

 

利用id(x)，查看下x += [1]對象是怎么變化的吧：
x = [1,2,3]  
print "before func(), global! x = ",x,"id(x) = ",id(x)  
  
def func(x):  
    print "in func(), local! original x = ",x,"id(x) = ",id(x)  
    x += [1]  
    print "in func(), local! now x = ",x,"id(x) = ",id(x)  
  
func(x)  
print "after func(), global! x = ",x,"id(x) = ",id(x)  
結果：
before func(), global! x =  [1, 2, 3] id(x) =  46536584  
in func(), local! original x =  [1, 2, 3] id(x) =  46536584  
in func(), local! now x =  [1, 2, 3, 1] id(x) =  46536584  
after func(), global! x =  [1, 2, 3, 1] id(x) =  46536584  
id(x)全程一樣，x += [1]，python直接就在原對象上操作

 

 

參考：

1、http://blog.csdn.net/emaste_r/article/details/47395055