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

Question

Hi guys,

So I am relatively new to Python and I am having trouble working with 2D Lists.

Here's my code:

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

and here is the output (formatted for readability):

[['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 every row get assigned the value?

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 does what you expect:

>>> 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

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

Answer 3

As the python library reference on * for lists says

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]]