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

Question

Possible Duplicate:
Unexpected feature in a Python list of lists

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]]