Unexpected output of python list

Question

I am new to python and I am trying to run following code:

m=int(raw_input())
l=[[0]*2]*m
for i in range(0,m):
    for j in range(0,2):
        l[i][j]=raw_input()
        print l
        print "-------"

Output:

3
1
[['1', 0], ['1', 0], ['1', 0]]
-------
2
[['1', '2'], ['1', '2'], ['1', '2']]
-------
3
[['3', '2'], ['3', '2'], ['3', '2']]
-------
4
[['3', '4'], ['3', '4'], ['3', '4']]
-------
5
[['5', '4'], ['5', '4'], ['5', '4']]
-------
6
[['5', '6'], ['5', '6'], ['5', '6']]

but, when i dry run the program the expected output should be

3
1
[['1', 0], ['1', 0], ['1', 0]]
-------
2
[['1', '2'], ['1', '2'], ['1', '2']]
-------
3
[['1', '2'], ['3', '2'], ['1', '2']]
-------
4
[['1', '2'], ['3', '4'], ['1', '2']]
-------
5
[['1', '2'], ['3', '4'], ['5', '2']
-------
6
[['1', '2'], ['3', '4'], ['5', '6']]

Its like after each iteration list is reassigned, but as I am incrementing 'i' list elements should be like the expected output. Please i need help..

kev · Accepted Answer · 2012-03-11 10:12:58Z

6

s * n, n * s    ==>   n shallow copies of s concatenated

Note 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]]

You can change your code to:

l = [[0 for i in range(2)] for j in range(m)]

answered Mar 11, 2012 at 10:12

kev

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3 Comments

sum2000 Over a year ago

but the variable i is ranging from 0 to m, so in first iteration first element of list l , i.e l[0][0]=1 and l[0][1]=2 is correct, but in second iteration l[1][0]=3 and l[1][1]=4, but why the values of l[0][0] and l[0][1] are changed in second iteration of 'i'..?

user395760 Over a year ago

@sum2000 Becaus ( as he states) the inner lists are not copied. Your l is simply a list storing m references to the same 2-element list.

kev Over a year ago

l[0][0], l[1][0], l[2][0] point to the same object in memory.

aquavitae · Accepted Answer · 2012-03-11 10:20:36Z

The problem in the line l=[[0]*2]*m. Doing [[0]] * 3 results in 3 * shallow copies* of the list [0], not three separate lists. A better way of writing your code would be to construct the list as you go. Note that you don't need to specify the start of the range as 0. Also, in python2, xrange is much faster than range, so tis good practice to use it instead.

m=int(raw_input())
l=[]
for i in xrange(m):
    l.append([])
    for j in xrange(2):
        l[i].append(raw_input())
        print l
        print "-------"

You could also do it using a generator and iterating over the list:

m=int(raw_input())
l=[[0, 0] for _ in range(m)]
for sub_list in l:
    for j in xrange(2):
        sub_list[j] = raw_input()
        print l
        print "-------"

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