python中a=a+2与a+=2的区别
Posted 沧海一粟,何以久远
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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
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