Lists in Python: Create, Traverse, Change and List Methods

Creating a list

A list is an ordered collection of items in [ ], separated by commas.

marks = [72, 85, 90]
mix = ["Ravi", 16, 7.5, True]   # different types allowed
empty = []
nested = [1, [2, 3]]            # a list inside a list
L = list("abc")                 # ["a", "b", "c"]
nums = list(range(1, 6))        # [1, 2, 3, 4, 5]

You can also build one from input: L = eval(input()) or by appending in a loop.

Indexing and slicing

Each item has an index: 0, 1, 2… from the left, and −1, −2… from the right.

L[i] gives one item. A wrong index gives IndexError.

Slicing L[start:stop:step] gives a new list from start up to stop − 1. L[:2] = first two, L[2:] = from index 2 on, L[::-1] = reversed copy.

Traversing a list

Visiting each item once is called traversal.

for x in L:              # by item
    print(x)

for i in range(len(L)):  # by index
    print(i, L[i])

Use the index form when you need the position or want to change items.

Changing a list and list operators

Lists are mutable: L[2] = 35 changes an item; L[0:2] = [1, 1] changes a slice; del L[1] deletes an item.

• + joins: [1, 2] + [3] → [1, 2, 3]
• * repeats: [0] * 3 → [0, 0, 0]
• in / not in checks membership
• ==, < compare item by item

Built-in functions for lists

• len(L): number of items
• max(L), min(L): largest, smallest
• sum(L): total of numbers
• sorted(L): returns a NEW sorted list, L is unchanged
• list(x): makes a list from a string, tuple or range

List methods

• append(x): add one item at the end
• extend(list2): add every item of another list at the end
• insert(i, x): put x at index i
• remove(x): delete the first x (ValueError if missing)
• pop(i): delete and return item at i (default last)
• clear(): empty the list
• index(x): position of first x
• count(x): how many times x appears
• sort(): sort in place (reverse=True for descending)
• reverse(): reverse in place
• copy(): a new copy

Most methods change the list and return None. So L = L.sort() makes L None!

Try it: class marks list

Write the marks of 5 friends in a list. In Python: print the highest, lowest and average (sum(L)/len(L)). Append a new friend's marks, sort the list, then remove the lowest with L.pop(0). Predict each result first, then check with step 6 of the 3D.

Key formulas and definitions

• L[i], L[-i]; slice L[start:stop:step] (stop not included)
• len, max, min, sum, sorted, list
• append, extend, insert, remove, pop, clear, index, count, sort, reverse, copy
• average = sum(L) / len(L)

Worked examples

Common mistakes

• Thinking the first index is 1. It is 0.
• Writing L = L.sort(); sort() returns None.
• Mixing pop (by position) and remove (by value).
• Using append to add many items; use extend.