Python中a和a[:]有什么区别?

• []是引用 传址调用
• [:] 是复制 传值调用

发现用b=a[:], 再给b赋值, 不会影响a; 直接给a[:]赋值却会影响a

a=5

b=a

print(a,b)

b=3

print(a,b)

a=6

print(a,b)

5 5
5 3
6 3

c=[1,2,3,4]

d=c

print(c,d)

c[0]=5

print(c,d)

d[0]=6

print(c,d)

[1, 2, 3, 4] [1, 2, 3, 4]
[5, 2, 3, 4] [5, 2, 3, 4]
[6, 2, 3, 4] [6, 2, 3, 4]

c=[1,2,3,4]

d=c[:]

print(c,d)

c[0]=5

print(c,d)

d[0]=6

print(c,d)

[1, 2, 3, 4] [1, 2, 3, 4]
[5, 2, 3, 4] [1, 2, 3, 4]
[5, 2, 3, 4] [6, 2, 3, 4]

c=[1,2,[1,2,3],3,4]

d=c[:] #  没有限制条件的分片表达式（L[:]）能够复制序列，但此法只能浅层复制; 在有嵌套情况跟着一起变化

print(c,d)

c[0]=5

c[2][0]=7

print(c,d)

d[0]=6

print(c,d)

[1, 2, [1, 2, 3], 3, 4] [1, 2, [1, 2, 3], 3, 4]
[5, 2, [7, 2, 3], 3, 4] [1, 2, [7, 2, 3], 3, 4]
[5, 2, [7, 2, 3], 3, 4] [6, 2, [7, 2, 3], 3, 4]

import copy

c=[1,2,3,4]

d=copy.deepcopy(c)

print(c,d)

c[0]=5

print(c,d)

d[0]=6

print(c,d)

[1, 2, 3, 4] [1, 2, 3, 4]
[5, 2, 3, 4] [1, 2, 3, 4]
[5, 2, 3, 4] [6, 2, 3, 4]

a = [0, [1, 2], 3]

b = a[:]

a[0] = 8

a[1][1] = 9

print(a,b)

[8, [1, 9], 3] [0, [1, 9], 3]

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import copy

a = [0, [1, 2], 3]

b = copy.deepcopy(a)

a[0] = 8

a[1][1] = 9

print(a,b)

[8, [1, 9], 3] [0, [1, 2], 3]