Learning python - List Data Type

The List Data Type

A list is a collection of items stored in a specific order. Think of it as a row of boxes, where each box can hold a value. Unlike a variable that stores just one thing, a list can store many.

Lists are written with square brackets, and items are separated by commas:

numbers = [10, 20, 30, 40]
names = ["Alice", "Bob", "Charlie"]
mixed = [1, "hello", 3.5]

Lists are useful because you can add, remove, and change items whenever you need to. This makes them one of the most common data types in Python.

Example: Using a list with `sum()`

# List of daily steps taken over a week
daily_steps = [6540, 7120, 8030, 4990, 10220, 8870, 7600]

# Use sum() to add all values in the list
total_steps = sum(daily_steps)

# Print the total
print(total_steps)

More about the `sum()` function

sum() is a built-in Python function that adds together all the numbers in a list. It saves you from having to write your own loop.

numbers = [10, 20, 30]
total = sum(numbers)
print(total)
# Result: 60

The list must contain numbers (integers or floats). If Python can’t add the items together, it will show an error.

Common mistakes with `sum()`

Mixing data types

values = [10, "20", 30]
sum(values)  # Error: Python can’t add text and numbers

Using it on strings

names = ["Alice", "Bob"]
sum(names)  # Error: sum() cannot add text together

Trailing commas creating unintended lists

numbers = [10, 20, 30,]  # This is fine
numbers = [10, 20, "30",]  # This will break sum()

If you stick to a list of numbers, sum() works perfectly every time.

Example: Adding, removing, and inserting items

This example shows how websites might track recently viewed items:

# A list of recently viewed items on a website
recently_viewed = ["Laptop", "Headphones", "Desk Chair"]

# Add a new item the user just clicked on
recently_viewed.append("Monitor")

# Insert an item that was viewed again, so it moves to the top
recently_viewed.insert(0, "Laptop Stand")

# Remove an item that the system decides to clear out (oldest view)
recently_viewed.remove("Headphones")

# Show the first two items for a small homepage preview
preview_items = recently_viewed[:2]

# Print the final list and the preview
print(recently_viewed)
print(preview_items)

# Step-by-step result:
# Start: ["Laptop", "Headphones", "Desk Chair"]
# After append("Monitor"): ["Laptop", "Headphones", "Desk Chair", "Monitor"]
# After insert(0, "Laptop Stand"): ["Laptop Stand", "Laptop", "Headphones", "Desk Chair", "Monitor"]
# After remove("Headphones"): ["Laptop Stand", "Laptop", "Desk Chair", "Monitor"]
# preview_items = ["Laptop Stand", "Laptop"]

# Final output:
# ["Laptop Stand", "Laptop", "Desk Chair", "Monitor"]
# ["Laptop Stand", "Laptop"]

append() vs insert()

Both methods add new items to a list, but they place them in different positions.

append()

Adds an item to the end of the list. Use this when you simply want to add something new without changing the order.

items = ["a", "b", "c"]
items.append("d")
# Result: ["a", "b", "c", "d"]

insert()

Adds an item at a specific position. You choose the index to place the item (position 0 is the start).

items = ["a", "b", "c"]
items.insert(1, "x")
# Result: ["a", "x", "b", "c"]

Quick summary

append() → adds to the end
insert() → adds wherever you tell it

Vivienne Reay