Python · Lists

A list is an ordered, mutable collection, and it can hold items of different types. The operations I worked through today:

• append(x) adds one item. extend(iterable) adds every item from another iterable.
• insert(i, x), pop(i) and remove(x) insert or delete items.
• sort() sorts in place and returns None. sorted(lst) returns a new list and leaves the original alone.
• key= sorts by a computed value, for example key=len.

The biggest trap is aliasing. b = a does not copy the list. It gives the same list a second name, so changing b also changes a. Use a.copy() or a[:] to get a real copy (a shallow one).

a = [3, 1, 2]
a.append(4)
a.extend([0, 9])
print(a)

print(sorted(a), a)          # sorted() leaves a alone
print(a.sort(), a)           # sort() returns None, changes a

b = a          # alias
c = a.copy()   # copy
b.append(100)
print("a:", a)
print("c:", c)

words = ["numpy", "c++", "python", "ai"]
print(sorted(words, key=len))
print(sorted(words, key=len, reverse=True))

Output:

[3, 1, 2, 4, 0, 9]
[0, 1, 2, 3, 4, 9] [3, 1, 2, 4, 0, 9]
None [0, 1, 2, 3, 4, 9]
a: [0, 1, 2, 3, 4, 9, 100]
c: [0, 1, 2, 3, 4, 9]
['ai', 'c++', 'numpy', 'python']
['python', 'numpy', 'c++', 'ai']