Lists

Lists are mutable, ordered sequences: indexing and slicing, + and *, the list methods and what they return, nested lists, copying, and the standard list programs.

Key ideas

Lists are mutable: L[i] = x changes an item in place; indexing and slicing work as they do for strings.
append(x) adds one item at the end, extend(L2) adds every item of L2, and insert(i, x) puts x at index i.
remove(x) deletes the first x (a ValueError if there is none); pop() removes and gives back the last item, pop(i) the item at index i.
append, extend, insert, remove, sort and reverse change the list and give back None, so L = L.sort() leaves L as None.
sorted(L) gives a new sorted list and leaves L as it was; L.sort() sorts L itself.
M = L gives a second name for the same list; L.copy(), list(L) or L[:] makes a separate copy.
In a nested list M[i][j] is row i, column j, both counted from 0.

Remember

L = [10, 20, 30, 40]	L[1] = 20, L[-1] = 40, L[1:3] = [20, 30], L[::-1] = [40, 30, 20, 10]
Operators	[1, 2] + [3] = [1, 2, 3]; [0] * 3 = [0, 0, 0]; 3 in [1, 2, 3] is True
Methods	L = [5, 2, 8] L.append(1) # [5, 2, 8, 1] L.insert(0, 7) # [7, 5, 2, 8, 1] print(L.pop()) # 1 L.sort() # [2, 5, 7, 8]
append or extend	L.append([3, 4]) adds one item, a list; L.extend([3, 4]) adds 3 and then 4
Functions	len(L), sum(L), max(L), min(L); L.count(x); L.index(x) gives the first position
Mean of a list	sum(L) / len(L) is a float: [4, 5, 9] gives 6.0
Copy or not	M = L; M.append(9) changes L too; with M = L[:] it does not

Watch out

Writing L = L.sort(): sort() gives back None.
Mixing up remove(x), which takes a value, with pop(i), which takes an index.
Thinking M = L copies the list: both names share one list.
Using append() to add several items: append([3, 4]) adds one list inside the list.

Worked example

What does this program print?
L = [47, 9, 35, 15, 31, 38, 58, 39]
for i in range(0, len(L) - 1, 2):
L[i], L[i + 1] = L[i + 1], L[i]
print(L)

Working: items at even and odd positions swap in pairs; a last odd item stays

Answer: [9, 47, 15, 35, 38, 31, 39, 58]