了解dict.copy()-浅还是深? [英] Understanding dict.copy() - shallow or deep?

问题描述

在阅读dict.copy()的文档时，它说它制作了字典的浅表副本.我正在关注的书(Beazley的Python参考)也是如此，该书说:

While reading up the documentation for dict.copy(), it says that it makes a shallow copy of the dictionary. Same goes for the book I am following (Beazley's Python Reference), which says:

m.copy()方法变浅 包含在其中的项目的副本 映射对象并将其放置在 新的映射对象.

The m.copy() method makes a shallow copy of the items contained in a mapping object and places them in a new mapping object.

考虑一下:

>>> original = dict(a=1, b=2)
>>> new = original.copy()
>>> new.update({'c': 3})
>>> original
{'a': 1, 'b': 2}
>>> new
{'a': 1, 'c': 3, 'b': 2}

所以我认为这也会更新original的值(并添加'c':3)，因为我正在执行浅表复制.就像您对列表进行操作一样:

So I assumed this would update the value of original (and add 'c': 3) also since I was doing a shallow copy. Like if you do it for a list:

>>> original = [1, 2, 3]
>>> new = original
>>> new.append(4)
>>> new, original
([1, 2, 3, 4], [1, 2, 3, 4])

这按预期工作.

由于两者都是浅表副本，为什么dict.copy()不能按我预期的那样工作?还是我对浅复制和深复制的理解存在缺陷?

Since both are shallow copies, why is that the dict.copy() doesn't work as I expect it to? Or my understanding of shallow vs deep copying is flawed?

推荐答案

通过浅复制"，这意味着字典的 content 不会按值复制，而只是创建一个新引用. /p>

By "shallow copying" it means the content of the dictionary is not copied by value, but just creating a new reference.

>>> a = {1: [1,2,3]}
>>> b = a.copy()
>>> a, b
({1: [1, 2, 3]}, {1: [1, 2, 3]})
>>> a[1].append(4)
>>> a, b
({1: [1, 2, 3, 4]}, {1: [1, 2, 3, 4]})

相比之下，深层副本将按值复制所有内容.

In contrast, a deep copy will copy all contents by value.

>>> import copy
>>> c = copy.deepcopy(a)
>>> a, c
({1: [1, 2, 3, 4]}, {1: [1, 2, 3, 4]})
>>> a[1].append(5)
>>> a, c
({1: [1, 2, 3, 4, 5]}, {1: [1, 2, 3, 4]})

所以:

1. b = a:引用分配，使a和b指向同一对象.

1. b = a: Reference assignment, Make a and b points to the same object.

b = a.copy():浅复制，a和b将成为两个孤立的对象，但它们的内容仍共享相同的引用

b = a.copy(): Shallow copying, a and b will become two isolated objects, but their contents still share the same reference

b = copy.deepcopy(a):深层复制，a和b的结构和内容完全隔离.

b = copy.deepcopy(a): Deep copying, a and b's structure and content become completely isolated.

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