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), ...

Answer 1

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