How to deep copy a list?

Question

After E0_copy = list(E0), I guess E0_copy is a deep copy of E0 since id(E0) is not equal to id(E0_copy). Then I modify E0_copy in the loop, but why is E0 not the same after?

E0 = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for k in range(3):
    E0_copy = list(E0)
    E0_copy[k][k] = 0
    #print(E0_copy)
print E0  # -> [[0, 2, 3], [4, 0, 6], [7, 8, 0]]

Answer 1

E0_copy is not a deep copy. You don't make a deep copy using list(). (Both list(...) and testList[:] are shallow copies, as well as testList.copy().)

You use copy.deepcopy(...) for deep copying a list.

copy.deepcopy(x[, memo])

Return a deep copy of x.

See the following snippet -

>>> a = [[1, 2, 3], [4, 5, 6]]
>>> b = list(a)
>>> a
[[1, 2, 3], [4, 5, 6]]
>>> b
[[1, 2, 3], [4, 5, 6]]
>>> a[0][1] = 10
>>> a
[[1, 10, 3], [4, 5, 6]]
>>> b   # b changes too -> Not a deepcopy.
[[1, 10, 3], [4, 5, 6]]

Now see the deepcopy operation

>>> import copy
>>> b = copy.deepcopy(a)
>>> a
[[1, 10, 3], [4, 5, 6]]
>>> b
[[1, 10, 3], [4, 5, 6]]
>>> a[0][1] = 9
>>> a
[[1, 9, 3], [4, 5, 6]]
>>> b    # b doesn't change -> Deep Copy
[[1, 10, 3], [4, 5, 6]]

To explain, list(...) does not recursively make copies of the inner objects. It only makes a copy of the outermost list, while still referencing the same inner lists, hence, when you mutate the inner lists, the change is reflected in both the original list and the shallow copy. You can see that shallow copying references the inner lists by checking that id(a[0]) == id(b[0]) where b = list(a).