Python Lists and List Methods

A list is an ordered, mutable collection that can hold any mix of values and grow or shrink as needed. It is the workhorse data structure in Python: shopping carts, rows from a file, search results, queues of work.

This lesson covers creating lists, indexing and slicing, modifying lists, every list method — append, extend, insert, remove, pop, clear, index, count, sort, reverse, copy — and the difference between shallow and deep copies.

Creating, Indexing and Slicing

Create lists with brackets [1, 2, 3], list(iterable) or comprehensions. Indexes start at 0; negative indexes count from the end (items[-1] is the last item). Slices items[start:stop:step] return new lists; items[::-1] is a reversed copy. Lists can be nested to form tables.

Adding and Removing Items

append(x) adds one item at the end; extend(iterable) adds many; insert(i, x) inserts at a position. remove(x) deletes the first matching value (ValueError if absent); pop(i) removes and returns an item (the last by default); clear() empties the list; del items[i] or del items[a:b] deletes by index or slice.

Searching, Counting and Sorting

index(x) returns the first position of a value; count(x) counts occurrences; x in items tests membership. sort() sorts in place (with key= and reverse=True) and returns None; the built-in sorted() returns a new list. reverse() reverses in place.

Copying Lists

b = a does not copy — both names refer to the same list. a.copy(), a[:] and list(a) make a shallow copy: a new outer list whose inner objects are shared. For nested lists, use copy.deepcopy().

Examples

nums = [10, 20, 30, 40, 50, 60]
print(nums[0], nums[-1], nums[2:5], nums[:3], nums[::2], nums[::-1])

matrix = [[1, 2, 3], [4, 5, 6]]
print(matrix[1][2], [row[0] for row in matrix])
print(len(nums), 30 in nums, min(nums), max(nums), sum(nums))

10 60 [30, 40, 50] [10, 20, 30] [10, 30, 50] [60, 50, 40, 30, 20, 10]
6 [1, 4]
6 True 10 60 210

cart = ["pen", "book"]
cart.append("bag");              print("append :", cart)
cart.extend(["ink", "pen"]);     print("extend :", cart)
cart.insert(1, "ruler");         print("insert :", cart)
cart.remove("pen");              print("remove :", cart)
last = cart.pop();               print("pop    :", last, cart)
first = cart.pop(0);             print("pop(0) :", first, cart)
print("index  :", cart.index("bag"), "count:", cart.count("ink"))
cart.sort();                     print("sort   :", cart)
cart.sort(key=len, reverse=True); print("sort key:", cart)
cart.reverse();                  print("reverse:", cart)
backup = cart.copy()
cart.clear();                    print("clear  :", cart, "backup:", backup)

append : ['pen', 'book', 'bag']
extend : ['pen', 'book', 'bag', 'ink', 'pen']
insert : ['pen', 'ruler', 'book', 'bag', 'ink', 'pen']
remove : ['ruler', 'book', 'bag', 'ink', 'pen']
pop    : pen ['ruler', 'book', 'bag', 'ink']
pop(0) : ruler ['book', 'bag', 'ink']
index  : 1 count: 1
sort   : ['bag', 'book', 'ink']
sort key: ['book', 'bag', 'ink']
reverse: ['ink', 'bag', 'book']
clear  : [] backup: ['ink', 'bag', 'book']

students = [("Asha", 91), ("Ravi", 72), ("Meera", 91), ("Kiran", 85)]
ranked = sorted(students, key=lambda s: (-s[1], s[0]))
print(ranked)
print(students[0])              # original unchanged

nums = [5, 3, 8]
print(nums.sort())              # sort() returns None!
print(nums)

letters = list("abcdef")
letters[1:3] = ["X", "Y", "Z"]
del letters[-2:]
print(letters)

[('Asha', 91), ('Meera', 91), ('Kiran', 85), ('Ravi', 72)]
('Asha', 91)
None
[3, 5, 8]
['a', 'X', 'Y', 'Z', 'd']

import copy

a = [[1, 2], [3, 4]]
alias = a
shallow = a.copy()
deep = copy.deepcopy(a)

a[0].append(99)
a.append([5])
print("alias  :", alias)
print("shallow:", shallow)
print("deep   :", deep)

alias  : [[1, 2, 99], [3, 4], [5]]
shallow: [[1, 2, 99], [3, 4]]
deep   : [[1, 2], [3, 4]]