Python: list of lists


Question

Running the code

listoflists = []
list = []
for i in range(0,10):
    list.append(i)
    if len(list)>3:
        list.remove(list[0])
        listoflists.append((list, list[0]))
print listoflists

returns

[([7, 8, 9], 0), ([7, 8, 9], 0), ([7, 8, 9], 0), ([7, 8, 9], 1), ([7, 8, 9], 2), ([7, 8, 9], 3), ([7, 8, 9], 4), ([7, 8, 9], 5), ([7, 8, 9], 6), ([7, 8, 9], 7)]

so somehow the first argument of each tuple (list) is being updated each time in the list of lists, but the second argument list[0] is not. Can someone explain what's going on here and suggest a way to fix this? I'd like to output

[([0],0), ([0,1],0), ...
1
32
7/14/2012 8:09:44 PM

Accepted Answer

Lists are a mutable type - in order to create a copy (rather than just passing the same list around), you need to do so explicitly:

    listoflists.append((list[:], list[0]))

However, list is already the name of a Python built-in - it'd be better not to use that name for your variable. Here's a version that doesn't use list as a variable name, and makes a copy:

listoflists = []
a_list = []
for i in range(0,10):
    a_list.append(i)
    if len(a_list)>3:
        a_list.remove(a_list[0])
        listoflists.append((list(a_list), a_list[0]))
print listoflists

Note that I demonstrated two different ways to make a copy of a list above: [:] and list().

The first, [:], is creating a slice (normally often used for getting just part of a list), which happens to contain the entire list, and thus is effectively a copy of the list.

The second, list(), is using the actual list type constructor to create a new list which has contents equal to the first list. (I didn't use it in the first example because you were overwriting that name in your code - which is a good example of why you don't want to do that!)

42
7/14/2012 8:19:27 PM

I came here because I'm new with python and lazy so I was searching an example to create a list of 2 lists, after a while a realized the topic here could be wrong... this is a code to create a list of lists:

listoflists = []
for i in range(0,2):
    sublist = []
    for j in range(0,10)
        sublist.append((i,j))
    listoflists.append(sublist)
print listoflists

this the output [ [(0, 0), (0, 1), (0, 2), (0, 3), (0, 4), (0, 5), (0, 6), (0, 7), (0, 8), (0, 9)], [(1, 0), (1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (1, 7), (1, 8), (1, 9)] ]

The problem with your code seems to be you are creating a tuple with your list and you get the reference to the list instead of a copy. That I guess should fall under a tuple topic...


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