创建不引用包含对象的列表副本

问题描述

假设我有一个向量 a 定义为:

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

我了解到要创建对象 a 的副本而不引用它，我应该使用以下语法:

b = a[:]

确实，如果我执行以下语句:

b = []打印一个

输出是

>>>[[1,2,3],[-1,-2,-3]]

正如我所料.不过，如果我执行以下操作:

b = a[:]b[0][2] = '改变一个'打印一个

输出是

>>>[[1,2,'改变一个'],[-1,-2,-3]]

所以我很清楚对象 a[0] 正在被引用，即使包含在 a 中.如何创建对象 a 的副本，即使它的所有内部对象都不会被引用?

解决方案

为此使用 deepcopy:

>>>从复制导入 deepcopy>>>b = 深拷贝(a)>>>b[0][2] = '改变一个'>>>打印一个[[1,2,3],[-1,-2,-3]]

Deepcopy:https://docs.python.org/2/library/copy.html#copy.deepcopy

扩展

Deepcopy 还会创建类实例的单独副本.请看下面的简单例子.

from copy import deepcopyA类:def __init__(self):self.val = 'A'>>>a = A()>>>b = 深拷贝(a)>>>b.val = 'B'>>>打印 a.val'一种'>>>打印 b.val'乙'

Say I have a vector a defined as:

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

I have learned that to create a copy of the object a without referencing it I should use the following syntaxis:

b = a[:]

Indeed, if I execute the following statements:

b = []
print a

the output is

>>> [[1,2,3],[-1,-2,-3]]

exactly as I was expecting. Though, if I do the following:

b = a[:]
b[0][2] = 'change a'
print a

the output is

>>> [[1,2,'change a'],[-1,-2,-3]]

So it's clear to me that the object a[0] is being referenced even if contained in a. How can I create a copy of the object a in a way that even all its internal objects will not be referenced?

解决方案

For that use deepcopy:

>>> from copy import deepcopy
>>> b = deepcopy(a)

>>> b[0][2] = 'change a'
>>> print a
[[1,2,3],[-1,-2,-3]]

Deepcopy: https://docs.python.org/2/library/copy.html#copy.deepcopy

Extension

Deepcopy also creates an individual copy of class instances. Please see simple example below.

from copy import deepcopy

class A:
    def __init__(self):
        self.val = 'A'

>>> a = A()
>>> b = deepcopy(a)

>>> b.val = 'B'
>>> print a.val
'A'
>>> print b.val
'B'

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