python知识点：可变对象和不可变对象

python中，对象分为可变(mutable)和不可变(immutable)两种类型。

元组（tuple)、数值型（number){int,float}、字符串(string)均为不可变对象，而字典型(dictionary)和列表型(list)的对象是可变对象。
{我对可变对象的理解就是，变量a将其赋值给另一个变量b，修改这个变量b会直接修改原变量a，而不可变对象，就会复制变量a进行修改，不会修改原变量a；即可变对象引用指向一个，而不可变对象，随着修改，会指向不同的引用}

#coding:utf-8;

i_int = 10
j_int = 10
k_int = 11

i_str = 'Python'
j_str = 'Python'
k_str = 'python world'

i_tuple = (1,2)
j_tuple = (1,2)
k_tuple = (1,2,3)

print "i_int's id",str(id(i_int))
print "j_int's id",str(id(j_int))
print "k_int's id",str(id(k_int))

print "i_str's id",str(id(i_str))
print "j_str's id",str(id(j_str))
print "k_str's id",str(id(k_str))

print "i_tuple's id",str(id(i_tuple))
print "j_tuple's id",str(id(j_tuple))
print "k_tuple's id",str(id(k_tuple))

output:

i_int's id 30498944
j_int's id 30498944
k_int's id 30498920
i_str's id 140092758495168
j_str's id 140092758495168
k_str's id 140092757760912
i_tuple's id 140092757580688
j_tuple's id 140092757580760
k_tuple's id 140092757698192

发现了int,str的指向的值是一样的，那么它们的内存id一致；
值得注意的是tuple虽然都是（1,2），但内存id是不一样的；

j_int = j_int + 1
j_str = j_str + str("Add something")

print "Modified:j_int's id",str(id(j_int))
print "Modified:j_str's id",str(id(j_str))

output：

Modified:j_int's id 30498920
Modified:j_str's id 140092757761304

可以看到j_int和j_str的id已经改变，不再指向原来id。

#immutable
tmp = j_int

print "tmp's id",str(id(tmp))
tmp = tmp + 1
print "Modified:tmp's id",str(id(tmp))
print "j_int",j_int
print "tmp",tmp


#mutable
dt = {1:'1',2:'2'}
d = dt

print "dt's id",str(id(dt))
print "d's id",str(id(d))

d[3]='3'

print "dt's id",str(id(dt))
print "d's id",str(id(d))

for i in dt.keys():
    print "dt's key",i

for i in d.keys():
    print "d's key",i

output:

tmp's id 30498920
Modified:tmp's id 30498896
j_int 11
tmp 12
dt's id 140092757766224
d's id 140092757766224
dt's id 140092757766224
d's id 140092757766224
dt's key 1
dt's key 2
dt's key 3
d's key 1
d's key 2
d's key 3

参考：
http://www.jianshu.com/p/c5582e23b26c 解释不错