python中a=a+2与a+=2的区别

1、a=a+2，表示一个新的对象，新的对象名字还是a，但是指向的内存地址已经变了

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

所以对于tuple对象（不可变对象），也是可以这样操作的

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

2、a+=2对于有些对象的操作是表示原来的对象，对有些对象的操作是生成了一个新对象

不可变对象tuple1，操作完后，内存地址已经发生变化，生成一个新的对象
>>> tuple1=(1,2)
>>> type(tuple1)
<type 'tuple'>
>>> tuple1+=(3,)
>>> id(tuple1)
4521658880
>>> tuple1+=(4,5)
>>> id(tuple1)
4520649072

而list对象，可变对象，+=操作、append操作、extend操作，都是在原对象上操作

>>> 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)
结果：
[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就保证了，即使我的变量x在函数中指向对象变了，外部的x也会指向新的对象

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)
结果：
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]，是新建了一个对象，id(x) =  46390664

利用id(x)，查看下x += [1]对象是怎么变化的吧：
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)
结果：
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)全程一样，x += [1]，python直接就在原对象上操作

参考：

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