22. Lists

A list is one of Python's built-in data structures. It holds an ordered sequence of values — numbers, strings, booleans, or a mix — and lets you add, remove, and reorder items freely.

Creating a list

A list literal uses square brackets with commas between the values:

fruits = ["apple", "banana", "cherry"]
print(fruits[0])   # apple
print(fruits[1])   # banana
print(fruits[2])   # cherry

Important: Python lists are 0-indexed. The first item is at position 0, not 1. This is different from how we count in everyday life, so it trips up nearly every beginner at least once.

• fruits[0] is "apple" — the first item.
• fruits[1] is "banana" — the second item.
• fruits[2] is "cherry" — the third item.

A list with three items has valid positions 0, 1, and 2. Position 3 does not exist and raises an IndexError.

Length with len

len(t) returns the number of items in the list:

fruits = ["apple", "banana", "cherry"]
print(len(fruits))        # 3
print(fruits[len(fruits) - 1])   # cherry  (the last one)

Because the last valid index is always len(t) - 1, Python offers a shortcut: negative indices count from the end. t[-1] is always the last item:

print(fruits[-1])   # cherry
print(fruits[-2])   # banana

Looping over a list

Two natural ways to walk through a list.

for loop

The simplest loop gives you each value in order:

for item in fruits:
    print(item)

Output:

apple
banana
cherry

enumerate — index and value together

When you also need the position, use enumerate. By default it starts counting at 0:

for index, value in enumerate(fruits):
    print(index, value)

Output:

0 apple
1 banana
2 cherry

If you want 1-based display numbers (for user-facing output), pass a start argument:

for index, value in enumerate(fruits, 1):
    print(f"{index}. {value}")

Output:

1. apple
2. banana
3. cherry

Adding and removing items

Append to the end with .append:

fruits = ["apple", "banana"]

fruits.append("cherry")    # add to the end
print(fruits[-1])           # cherry

.insert(i, value) inserts at position i and shifts later items right. Remember: positions are 0-indexed, so position 0 is the very beginning:

fruits.insert(0, "apricot")   # new first item
print(fruits[0])               # apricot
print(fruits[1])               # apple  (shifted from 0 to 1)

.pop() removes and returns the last item. .pop(i) removes position i:

fruits.pop()     # removes the last item
fruits.pop(0)    # removes the first item

Lists are passed by reference

Lists behave differently from numbers and strings in one important way: assigning a list to another variable does not make a copy.

a = [1, 2, 3]
b = a       # b refers to the SAME list
b[0] = 99
print(a[0])   # 99   (the change shows through a too)

Both a and b name one list. For a real copy, use .copy() or the slice a[:]:

c = a.copy()
c[0] = 0
print(a[0])   # 99   (a is unchanged)

Homework

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