Sorting: sort vs sorted, reverse, key=

By the end of this lesson you can:

• Choose between list.sort() and sorted(list) and say why.
• Sort largest-first with reverse=True.
• Sort by your own rule with key=, such as length or a chosen field.

A very common first attempt. What does it print?

marks = [78, 91, 65]
result = marks.sort()

print(result)
print(marks)

None
[65, 78, 91]

Tidying a bookshelf versus writing out a tidy catalogue. One rearranges the shelf; the other leaves the shelf alone and produces a new list.

marks = [78, 91, 65, 88]

marks.sort()
print("sorted in place:", marks)

names = ["Ana", "Mia", "Leo"]
alphabetical = sorted(names)

print("new list: ", alphabetical)
print("original:", names)

Output

sorted in place: [65, 78, 88, 91]
new list:  ['Mia', 'Ana', 'Leo']
original: ['Ana', 'Mia', 'Leo']

• items.sort() — a method, changes the list, returns None. Use when the order you had does not matter.
• sorted(items) — a built-in, leaves the list alone, returns a new one. Use when you need both orders.

Backwards

marks = [78, 91, 65, 88]

print(sorted(marks, reverse=True))
print(sorted(["Ana", "Mia", "Leo"], reverse=True))

Output

[91, 88, 78, 65]
['Leo', 'Ana', 'Mia']

reverse=True works on both sort() and sorted(). It is not the same as items.reverse(), which flips the current order without sorting at all.

Sorting by your own rule

key= takes a function, and Python calls it on each item to decide what to compare. Note the missing brackets: you pass the function itself, not a call to it.

dishes = ["ice cream", "pizza", "hot dog", "pancake telur"]

print(sorted(dishes))
print(sorted(dishes, key=len))
print(sorted(dishes, key=len, reverse=True))

Output

['ice cream', 'pizza', 'pancake telur', 'hot dog']
['hot dog', 'ice cream', 'pizza', 'pancake telur']
['pancake telur', 'pizza', 'ice cream', 'hot dog']

The first is alphabetical; the others are by length, short-first then long-first. Same list, three orders, one argument.

Your own key function

Any function taking one item and returning something comparable will do.

def surname(full_name):
    """The last word of a name - what a register sorts by."""
    return full_name.split()[-1]


pupils = ["Mia Tom", "Leo Reed", "Ana Rivera", "Hugo Kumar"]

print(sorted(pupils))
print(sorted(pupils, key=surname))

Output

['Mia Tom', 'Hugo Kumar', 'Ana Rivera', 'Leo Reed']
['Hugo Kumar', 'Ana Rivera', 'Mia Tom', 'Leo Reed']

That is Section 1 · Functions earning its place: a six-word function turns a first-name sort into a surname sort.

Why it matters. Leaderboards, price lists, registers, search results — all sorted, and almost never by the plain alphabet.

Each dish is a small list: name, price, minutes to cook. Sorting by any of them is one key function.

# menu_sort.py - the same menu, ordered five ways

menu = [
    ["pizza", 3.50, 5],
    ["pancake", 2.00, 3],
    ["noodles", 5.00, 8],
    ["ice cream", 4.90, 2],
    ["hot dog", 12.00, 15],
]


def price_of(dish):
    return dish[1]


def minutes_of(dish):
    return dish[2]


def show(title, rows):
    print(title)
    for name, price, minutes in rows:
        print(f"  {name:<14}{price:>7.2f}{minutes:>5} min")
    print()


show("As typed", menu)
show("By name", sorted(menu))
show("Cheapest first", sorted(menu, key=price_of))
show("Dearest first", sorted(menu, key=price_of, reverse=True))
show("Quickest first", sorted(menu, key=minutes_of))

# The original is still in its first order - sorted() never touched it
print("First dish is still:", menu[0][0])

Output

As typed
  pizza       3.50    5 min
  pancake       2.00    3 min
  noodles       5.00    8 min
  ice cream           4.90    2 min
  hot dog           12.00   15 min

By name
  ice cream           4.90    2 min
  noodles       5.00    8 min
  pizza       3.50    5 min
  pancake       2.00    3 min
  hot dog           12.00   15 min

Cheapest first
  pancake       2.00    3 min
  pizza       3.50    5 min
  ice cream           4.90    2 min
  noodles       5.00    8 min
  hot dog           12.00   15 min

Dearest first
  hot dog           12.00   15 min
  noodles       5.00    8 min
  ice cream           4.90    2 min
  pizza       3.50    5 min
  pancake       2.00    3 min

Quickest first
  ice cream           4.90    2 min
  pizza       3.50    5 min
  pancake       2.00    3 min
  noodles       5.00    8 min
  hot dog           12.00   15 min

First dish is still: pizza

Two things this shows:

• Sorting a list of lists compares the first item, then the second to break ties. That is why "By name" works with no key at all.
• sorted() protects the original. Five different orders, and menu is untouched at the end — which is why the last line still prints pizza.

items.sort() reorders in place and returns None; sorted(items) returns a new list and leaves the original alone. reverse=True flips the order, and key= takes a function — passed without brackets — that says what to compare. Sorting parallel lists separately breaks them; keep each record together instead.