difference in list creation in python

Question

I am trying to create nested list of arbitruary size N X M

assume N = 3, M = 3 when I create a list like this

c = [0 for col in range(N)]   
c = [c for r in range(M)]  
print c

and execute

c[0][0]=1

I get

c = [[1, 0, 0], [1, 0, 0], [1, 0, 0]]

But when I create a list like this:

c = [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

and execute

c[0][0] = 1

I get

c = [[1, 0, 0], [0, 0, 0], [0, 0, 0]]

I am new in python, can somebody give me some idea ?

Answer 1

In list comprehension, you are creating a reference to every element in the list. That means that every element will be mutated when only one index is accessed and updated.

c = [0 for col in range(3)]
c = [c for r in range(3)]  

c[0][0] = "Test"

Output:

[['Test', 0, 0], ['Test', 0, 0], ['Test', 0, 0]]

Edit:

A nested list of arbitrary size:

import random

new_list = [[0 for i in range(random.randint(1, 10))] for b in range(random.randint(1, 10))]

Output:

[[0, 0, 0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0], [0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]]