它如何在Python的复制和内存系统中工作？

Question

How does it work for Python's memory reference?

When I tried a program on a matrix, I used nested list. Then I found the original data was mutated, even I copied it. I tried several way. Finally I used copy.deepcopy() to solve the problem. So what's the meaning of Python's copy and reference function. How could we avoid such s-(problems).

data = [
[1,0,0,1,0,0,1,1,1],
[1,0,1,0,0,0,0,1,0],
[1,1,0,0,0,0,0,1,0],
[1,0,1,0,0,0,0,1,0],
[1,0,0,1,0,0,1,1,1],
[0,0,0,0,0,0,0,0,0],
]
# data1 = data.copy() # нет
# data1 = data[:] # нет
# data1 = list(data) # нет
# data1 = list(data) # нет
# data1 = copy.copy(data) # нет
# for all of above five, data1 will change with data, even they differ from ID

For these five, we shall see:

data[0][0] = 0
print(id(data))
print(id(data1))
print(data)
print(data1)
# Out:
# 4450009608
# 4450115528
# [[0, 0, 0, 1, 0, 0, 1, 1, 1], [1, 0, 1, 0, 0, 0, 0, 1, 0], [1, 1, 0, 0, 0, 0, 0, 1, 0], [1, 0, 1, 0, 0, 0, 0, 1, 0], [1, 0, 0, 1, 0, 0, 1, 1, 1], [0, 0, 0, 0, 0, 0, 0, 0, 0]]
# [[0, 0, 0, 1, 0, 0, 1, 1, 1], [1, 0, 1, 0, 0, 0, 0, 1, 0], [1, 1, 0, 0, 0, 0, 0, 1, 0], [1, 0, 1, 0, 0, 0, 0, 1, 0], [1, 0, 0, 1, 0, 0, 1, 1, 1], [0, 0, 0, 0, 0, 0, 0, 0, 0]]

Thus we have to use deepcopy, such a little pit.

Answer 1

This is explained (with more detail) in the documentation of the copy module :

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original. A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

All the attempts you made in your first example only create shallow copies.

Answer 2

This data1 = data.copy() and this data1 = data[:] both make copies, but only one level deep . Below the first level, only references are copied. Which is why you are not seeing what you expect at the second level.

Answer 3

The id s of data and data1 will be different when for example using:

data1 = list(data)

But the inner lists will still point to the same objects and have the same id s, that's why you have to use deepcopy to copy the whole contents of the original data to new objects.