python: list manipulation

Question

I have a list L of objects (for what it's worth this is in scons). I would like to create two lists L1 and L2 where L1 is L with an item I1 appended, and L2 is L with an item I2 appended.

I would use append but that modifies the original list.

How can I do this in Python? (sorry for the beginner question, I don't use the language much, just for scons)

Answer 1

L1=list(L)

duplicates the list. I guess you can figure out the rest :)

Answer 2

L1 = L + [i1]
L2 = L + [i2]

That is probably the simplest way. Another option is to copy the list and then append:

L1 = L[:]       #make a copy of L
L1.append(i1)

Answer 3

You can make a copy of your list

>>> x = [1, 2, 3]
>>> y = list(x)
>>> y.append(4)
>>> y
[1, 2, 3, 4]
>>> z = list(x)
>>> z.append(5)
>>> z
[1, 2, 3, 5]

or use concatenation, which will make a new list

>>> x = [1, 2, 3]
>>> y = x + [4]
>>> z = x + [5]
>>> y
[1, 2, 3, 4]
>>> z
[1, 2, 3, 5]

The former is probably a touch more idiomatic/common, but the latter works fine in this case. Some people also copy using slicing (x[:] makes a new list with all the elements of the original list x referred to) or the copy module. Neither of these are awful, but I find the former a touch cryptic and the latter a bit silly.