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

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]]

As the python library reference for sequence types, which includes 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]]

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.