Python List Operations

Let’s Do Things with Lists

We know how to create a list. We know how to access items. We know how to add and remove items.

But there is more we can do with lists. We can combine them. Repeat them. Check if something is inside. Find the smallest or largest number. Add up all the numbers. And more.

These are called list operations. They are the everyday tools we use when working with lists.

Let’s go through them one by one.

Joining Lists with + in Python

We can join two lists using the + operator. This is called concatenation.

list1 = [1, 2, 3]
list2 = [4, 5, 6]
combined = list1 + list2
print(combined)

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

It creates a new list with all the items from both lists. The original lists are not changed.

We can also use += to add items to an existing list.

list1 = [1, 2, 3]
list1 += [4, 5]
print(list1)

[1, 2, 3, 4, 5]

That modifies list1 in place.

Repeating a List with * in Python

We can repeat a list using the * operator.

numbers = [1, 2]
repeated = numbers * 3
print(repeated)

[1, 2, 1, 2, 1, 2]

It repeats the list the given number of times. Again, it creates a new list.

This is useful when we want to initialize a list with the same value multiple times.

zeros = [0] * 5
print(zeros)

[0, 0, 0, 0, 0]

But let’s be careful when the list contains mutable objects like other lists. Because the repetition copies the reference, not the object.

Checking Membership with in

We can check if an item is in a list using in.

fruits = ["apple", "banana", "orange"]
print("banana" in fruits)

True

print("grape" in fruits)

False

We can also use not in to check the opposite.

print("grape" not in fruits)

True

This is very handy. It is one of the most common operations on lists.

Looping Through a List in Python

We already saw this in the introduction. But it is worth repeating because it is so common.

fruits = ["apple", "banana", "orange"]
for fruit in fruits:
    print(fruit)

apple
banana
orange

We can also loop with index using enumerate().

for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

0: apple
1: banana
2: orange

That gives us both the position and the value.

Finding the Length with len()

len() gives us the number of items in a list.

fruits = ["apple", "banana", "orange"]
print(len(fruits))

3

It works on any sequence. Lists, strings, tuples, dictionaries. Very useful.

Finding the Smallest and Largest in a Python List

Python has built-in functions min() and max() for lists of numbers (or any comparable items).

numbers = [5, 2, 9, 1, 7]
print(min(numbers))
print(max(numbers))

1
9

They also work on strings. It compares them alphabetically.

words = ["banana", "apple", "cherry"]
print(min(words))
print(max(words))

apple
cherry

Adding Up Numbers with sum()

sum() adds up all the numbers in a list.

numbers = [1, 2, 3, 4, 5]
print(sum(numbers))

15

It only works on numbers. If we try it on strings, we get an error.

words = ["a", "b"]
print(sum(words))   # Error!

Sorting with sorted()

sorted() returns a new list with the items sorted. The original list is not changed.

numbers = [5, 2, 9, 1, 7]
sorted_numbers = sorted(numbers)
print(sorted_numbers)

[1, 2, 5, 7, 9]

We can sort in reverse order.

print(sorted(numbers, reverse=True))

[9, 7, 5, 2, 1]

If we want to sort the list in place, we use the .sort() method. sorted() is a built-in function that works on any iterable.

Checking any() and all() in Python

any() returns True if at least one item is truthy.

numbers = [0, 0, 1, 0]
print(any(numbers))

True

Because there is a 1 (which is truthy).

all() returns True only if all items are truthy.

numbers = [1, 2, 3]
print(all(numbers))

True

numbers = [1, 0, 3]
print(all(numbers))

False

Because 0 is falsy.

These are useful for quick checks.

Python List Operations: A Real-Life Example

Let’s combine lists of names and check some things.

group1 = ["Alex", "Sam", "John"]
group2 = ["Emma", "David", "Sam"]

all_names = group1 + group2
print(f"All names: {all_names}")
print(f"Total names: {len(all_names)}")

if "Sam" in all_names:
    print("Sam is in the list")

unique_names = list(set(all_names))
print(f"Unique names: {unique_names}")

All names: ['Alex', 'Sam', 'John', 'Emma', 'David', 'Sam']
Total names: 6
Sam is in the list
Unique names: ['Alex', 'Sam', 'John', 'Emma', 'David']

That shows concatenation, length, membership, and even a quick way to remove duplicates using a set.

Common Mistakes with Python List Operations

numbers = [1, 2, 3]
numbers + 4   # Error!

numbers + [4]

numbers = [3, 1, 2]
sorted(numbers)
print(numbers)

[3, 1, 2]

numbers = sorted(numbers)

words = ["hello", "world"]
sum(words)   # Error!

mixed = [1, "hello", 3]
min(mixed)   # Error!

names = ["Alex", "Sam"]
print("alex" in names)

False