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]]