Python Sorting

1. Two Ways to Sort

2. .sort() — Sort In Place

Modifies the list directly. Returns None.

lst = [2, 1, 3, 4, 2, 3, 2, 1]

lst.sort()
print(lst)         # → [1, 1, 2, 2, 2, 3, 3, 4]

# ❌ Don't do this — it prints None, not the sorted list
print(lst.sort())  # → None

3. sorted() — Returns a New Sorted List

Does not change the original. Always returns a list.

lst = [2, 1, 3, 4, 2, 3, 2, 1]

new_lst = sorted(lst)
print(new_lst)     # → [1, 1, 2, 2, 2, 3, 3, 4]
print(lst)         # → [2, 1, 3, 4, 2, 3, 2, 1]  (unchanged)

4. Descending Order — reverse=True

Works with both .sort() and sorted().

lst.sort(reverse=True)             # in place, descending
sorted(lst, reverse=True)          # new list, descending

lst = [2, 1, 3, 4]
lst.sort(reverse=True)
print(lst)   # → [4, 3, 2, 1]

5. Sorting Tuples

Tuples don't have a .sort() method (they're immutable). Use sorted() instead — it returns a list.

tup = (3, 1, 2)

sorted(tup)               # → [1, 2, 3]  ← returns a list!
tuple(sorted(tup))        # → (1, 2, 3)  ← convert back to tuple

6. Custom Sort with key=

Pass a function to key= to sort by a custom value. The function receives each item and returns what to sort by.

lst = [[1, 2], [3, -6], [5, -4], [-1, 0], [-1, -2]]

# Sort by the second element of each inner list
def sort_second(item):
    return item[1]

lst.sort(key=sort_second)

⚠️ Pass the function name only — do not call it with ().

lst.sort(key=sort_second)     # ✅ correct
lst.sort(key=sort_second())   # ❌ wrong — this calls the function immediately

Sort by sum of inner list

def sort_by_sum(item):
    return sum(item)

lst.sort(key=sort_by_sum)

key= also works with sorted()

new_lst = sorted(lst, key=sort_second)
new_lst = sorted(lst, key=sort_second, reverse=True)

Cheat Sheet

lst = [3, 1, 4, 1, 5, 9]

# Sort in place (ascending)
lst.sort()

# Sort in place (descending)
lst.sort(reverse=True)

# Return new sorted list (ascending)
sorted(lst)

# Return new sorted list (descending)
sorted(lst, reverse=True)

# Sort tuple (returns list)
sorted((3, 1, 2))           # → [1, 2, 3]
tuple(sorted((3, 1, 2)))    # → (1, 2, 3)

# Custom sort with key
def my_key(item):
    return item[1]          # sort by second element

lst.sort(key=my_key)
sorted(lst, key=my_key, reverse=True)