In the following snippet, I append a list, x to another list u and the clear x. However, as you can see in the output, after clearing x, the second list, u is also cleared.

a = array([1,2,3,4])
x = []
u = []
for i in np.nditer(a):
    x.append(i.item())
u.append(x)
print("before x.clear(), u=", u)
x.clear()
print("after x.clear(), u=", u)

Output

before x.clear(), u= [[1, 2, 3, 4]]
after x.clear(), u= [[]]

What is the reason? I expect that by using append a copy of x is appended to u. How can I prevent u to be cleared after clearing x?

CodePudding user response：

Mark really answered this in the comments, but just for completeness, here's the working code using x.copy() doing what you asked for...

import numpy as np

a = np.array([1,2,3,4])
x = []
u = []
for i in np.nditer(a):
    x.append(i.item())
u.append(x.copy())
print("before x.clear(), u=", u)
x.clear()
print("after x.clear(), u=", u)

Output:

before x.clear(), u= [[1, 2, 3, 4]]
after x.clear(), u= [[1, 2, 3, 4]]

CodePudding user response：

I think you get it wrong. list.append() add an item to the end of the list. If you want to keep a copy of list and not affecting the copied one when you modified or deleted the original. You can do it several ways.

1. u = x[:] or
2. from copy import deepcopy

`u = deepcopy(x)`

See the documentation: https://docs.python.org/3/tutorial/datastructures.html#more-on-lists