Python：deepcopy（list）vs new_list = old_list [：] [英] Python: deepcopy(list) vs new_list = old_list[:]

问题描述

我从 http://openbookproject.net/thinkcs练习＃9 /python/english2e/ch09.html ，并遇到了没有意义的东西。

练习建议使用 copy

  .deepcopy（）使我的任务更容易， def add_row（matrix）：

>>> m = [[0,0]，[0,0]] 
>>> add_row m] 
 [[0,0]，[0,0]，[0,0]] 
>>> n = [[3,2,5]，[1,4 ，7]] 
>>> add_row（n）
 [[3,2,5]，[1,4,7]，[0,0,0]] 
>>> n 
 [[3,2,5]，[1,4,7]] 

 
 import copy 
 ＃end = copy.deepcopy（matrix）＃first way 
 final = matrix [：]＃second way 
 li = [] 
 for i in range（len（matrix [0]）） ：
 li.append（0）
＃return final.append（li）＃为什么这不工作？ 
 final.append（li）＃但这样做
 return final 
  

I当一个简单的列表[：] 复制它时，为什么本书建议使用 deepcopy（）我使用它错了吗？我的函数是否完全无效？

我也有一些混乱返回值。

解决方案

您提出了两个问题：

深与浅拷贝

matrix [：] 是浅拷贝 - 它仅复制直接存储在其中的元素，而不会递归地复制数组元素或其自身内的其他引用。这意味着：

  a = [[4]] 
b = a [：] 
a [0] .append（5）
 print b [0]＃输出[4，5]作为[0]和b [0]指向同一个数组
  

如果你在 a 中存储一个对象，同样会发生。

deepcopy（）当然是一个深拷贝 - 它递归地复制每个元素，

  a = [[4]] 
c = copy.deepcopy（a）
a [0] .append（5）
 print c [0]＃输出[4]，因为c [0]是将[0]的元素复制到新数组
  





返回

return final.append li）不同于调用 append 并返回 final ，因为list.append不返回列表对象本身，它返回 None

I'm doing exercise #9 from http://openbookproject.net/thinkcs/python/english2e/ch09.html and have ran into something that doesn't make sense.

The exercise suggests using copy.deepcopy() to make my task easier but I don't see how it could.

def add_row(matrix):
    """
        >>> m = [[0, 0], [0, 0]]
        >>> add_row(m)
        [[0, 0], [0, 0], [0, 0]]
        >>> n = [[3, 2, 5], [1, 4, 7]]
        >>> add_row(n)
        [[3, 2, 5], [1, 4, 7], [0, 0, 0]]
        >>> n
        [[3, 2, 5], [1, 4, 7]]
    """

    import copy
    # final = copy.deepcopy(matrix)  # first way
    final = matrix[:]                # second way
    li = []
    for i in range(len(matrix[0])):
        li.append(0)
    # return final.append(li)  # why doesn't this work?
    final.append(li)            # but this does
    return final

I'm confused why the book suggests using deepcopy() when a simple list[:] copies it. Am I using it wrong? Is my function completely out of wack?

I also have some confusion returning values. the question is documents in the code above.

TIA

解决方案

You asked two questions:

Deep vs. shallow copy

matrix[:] is a shallow copy -- it only copies the elements directly stored in it, and doesn't recursively duplicate the elements of arrays or other references within itself. That means:

a = [[4]]
b = a[:]
a[0].append(5)
print b[0] # Outputs [4, 5], as a[0] and b[0] point to the same array

The same would happen if you stored an object in a.

deepcopy() is, naturally, a deep copy -- it makes copies of each of its elements recursively, all the way down the tree:

a = [[4]]
c = copy.deepcopy(a)
a[0].append(5)
print c[0] # Outputs [4], as c[0] is a copy of the elements of a[0] into a new array

Returning

return final.append(li) is different from calling append and returning final because list.append does not return the list object itself, it returns None

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