Modifying list contents in Python

Question

I have a list like:

list = [[1,2,3],[4,5,6],[7,8,9]]

I want to append a number at the start of every value in the list programmatically, say the number is 9. I want the new list to be like:

list = [[9,1,2,3],[9,4,5,6],[9,7,8,9]]

How do I go about doing this in Python? I know it is a very trivial question but I couldn't find a way to get this done.

Answer 1

Use the insert method, which modifies the list in place:

>>> numberlists = [[1,2,3],[4,5,6]]
>>> for numberlist in numberlists:
...  numberlist.insert(0,9)
...
>>> numberlists
[[9, 1, 2, 3], [9, 4, 5, 6]]

or, more succintly

[numberlist.insert(0,9) for numberlist in numberlists]

or, differently, using list concatenation, which creates a new list

newnumberlists = [[9] + numberlist for numberlist in numberlists]

Answer 2

>>> someList = [[1,2,3],[4,5,6],[7,8,9]]
>>> someList = [[9] + i for i in someList]
>>> someList
[[9, 1, 2, 3], [9, 4, 5, 6], [9, 7, 8, 9]]

(someList because list is already used by python)

Answer 3

for sublist in thelist:
  sublist.insert(0, 9)

don't use built-in names such as list for your own stuff, that's just a stupid accident in the making -- call YOUR stuff mylist or thelist or the like, not list.

Edit: as the OP aks how to insert > 1 item at the start of each sublist, let me point out that the most efficient way is by assignment of the multiple items to a slice of each sublist (most list mutators can be seen as readable alternatives to slice assignments;-), i.e.:

for sublist in thelist:
  sublist[0:0] = 8, 9

sublist[0:0] is the empty slice at the start of sublist, and by assigning items to it you're inserting the items at that very spot.

Answer 4

#!/usr/bin/env python

def addNine(val):
    val.insert(0,9)
    return val

if __name__ == '__main__':
    s = [[1,2,3],[4,5,6],[7,8,9]]
    print map(addNine,s)

Output:

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

Answer 5

If you're going to be doing a lot of prepending,
perhaps consider using deques* instead of lists:

>>> mylist = [[1,2,3],[4,5,6],[7,8,9]]

>>> from collections import deque
>>> mydeque = deque()
>>> for li in mylist:
...   mydeque.append(deque(li))
...
>>> mydeque
deque([deque([1, 2, 3]), deque([4, 5, 6]), deque([7, 8, 9])])
>>> for di in mydeque:
...   di.appendleft(9)
...
>>> mydeque
deque([deque([9, 1, 2, 3]), deque([9, 4, 5, 6]), deque([9, 7, 8, 9])])

*Deques are a generalization of stacks and queues (the name is pronounced "deck" and is short for "double-ended queue"). Deques support thread-safe, memory-efficient appends and pops from either side of the deque with approximately the same O(1) performance in either direction.

And, as others have mercifully mentioned:
For the love of all things dull and ugly,
please do not name variables after your favorite data-structures.