Lists in Python — Create, Change, Sort and Slice

A list holds many values in order, in square brackets. Lists can grow, shrink and change — they are mutable.

marks = [78, 92, 65, 88]
print(marks[0], marks[-1], len(marks))

78 88 4

1. Changing a list

nums = [10, 20, 30]
nums.append(40)        # add at the end
nums.insert(0, 5)      # add at index 0
nums.remove(20)        # remove the first 20
last = nums.pop()      # remove and return the last item
nums[1] = 15           # replace by index
print(nums, last)

[5, 15, 30] 40

Other useful methods: extend(other_list), index(x), count(x), clear(), reverse().

2. Slicing (same as strings)

nums = [0, 1, 2, 3, 4, 5, 6]
print(nums[2:5], nums[:3], nums[-2:], nums[::-1])

[2, 3, 4] [0, 1, 2] [5, 6] [6, 5, 4, 3, 2, 1, 0]

3. Sorting

• lst.sort() sorts the list in place (returns None).
• sorted(lst) returns a new sorted list.

scores = [70, 95, 82, 60]
print(sorted(scores))
print(sorted(scores, reverse=True))
names = ["ravi", "Asha", "neha"]
print(sorted(names, key=str.lower))
students = [("Asha", 92), ("Ravi", 74), ("Neha", 85)]
print(sorted(students, key=lambda s: s[1], reverse=True))

[60, 70, 82, 95]
[95, 82, 70, 60]
['Asha', 'neha', 'ravi']
[('Asha', 92), ('Neha', 85), ('Ravi', 74)]

4. Useful built-ins

nums = [4, 8, 15, 16, 23, 42]
print(sum(nums), min(nums), max(nums))
print(sum(nums) / len(nums))
print(15 in nums, 99 in nums)

108 4 42
18.0
True False

5. Copying a list (a classic trap)

b = a does not copy — both names point to the same list (remember the name-tag picture).

a = [1, 2, 3]
b = a          # same list
c = a.copy()   # a real copy (also: a[:] or list(a))
a.append(4)
print(b)
print(c)

[1, 2, 3, 4]
[1, 2, 3]

6. Lists of lists (2D)

grid = [[1, 2, 3],
        [4, 5, 6]]
print(grid[1][2])
for row in grid:
    print(sum(row))

6
6
15

7. Common list programs

Second largest (without sorting)

nums = [12, 45, 7, 45, 30]
first = second = float("-inf")
for n in nums:
    if n > first:
        first, second = n, first
    elif first > n > second:
        second = n
print(second)

30

Remove duplicates but keep order

items = [3, 1, 3, 2, 1]
seen, unique = set(), []
for x in items:
    if x not in seen:
        seen.add(x)
        unique.append(x)
print(unique)

[3, 1, 2]

Common mistakes

• x = lst.sort() → x is None. Use sorted(lst) if you need the result.
• Thinking b = a copies a list.
• Index out of range: the last index is len(lst) - 1.

Quick recap

• Lists are ordered and mutable: append, insert, remove, pop, index assignment.
• Slicing works like strings.
• sort() in place vs sorted() new list; key= and reverse=.
• Copy with .copy(), [:] or list().