2D list has weird behavor when trying to modify a single value

Question

When I try this code:

data = [[None]*5]*5
data[0][0] = 'Cell A1'

The value of data ends up like:

[['Cell A1', None, None, None, None],
 ['Cell A1', None, None, None, None],
 ['Cell A1', None, None, None, None],
 ['Cell A1', None, None, None, None],
 ['Cell A1', None, None, None, None]]

Why does the 'Cell A1' value appear in every nested list?

Answer 1

This makes a list with five references to the same list:

data = [[None]*5]*5

Use something like this instead which creates five separate lists:

>>> data = [[None]*5 for _ in range(5)]

Now it behaves as expected:

>>> data[0][0] = 'Cell A1'
>>> print(data)
[['Cell A1', None, None, None, None],
 [None, None, None, None, None],
 [None, None, None, None, None],
 [None, None, None, None, None],
 [None, None, None, None, None]]

Answer 2

As explained in the documentation for sequence types (which includes lists):

Note also that the copies are shallow; nested structures are not copied. This often haunts new Python programmers; consider:

>>> lists = [[]] * 3
>>> lists
[[], [], []]
>>> lists[0].append(3)
>>> lists
[[3], [3], [3]]

What has happened is that [[]] is a one-element list containing an empty list, so all three elements of [[]] * 3 are (pointers to) this single empty list. Modifying any of the elements of lists modifies this single list. You can create a list of different lists this way:

>>> lists = [[] for i in range(3)]  
>>> lists[0].append(3)
>>> lists[1].append(5)
>>> lists[2].append(7)
>>> lists
[[3], [5], [7]]

Answer 3

In Python, every variable is an object, and thus a reference. You first created a list of 5 Nones, and then you build a list with 5 times the same object.