Python for Loops

Imagine printing a greeting for 500 customers by writing 500 print() lines. Nobody wants that. A Python for loop lets you write the code once and run it for every item in a collection. In this lesson, you'll learn how for loops work, how to loop over lists, strings and numbers, and how to use loops to add up totals, count things and build tables.

What is a for loop in Python?

A for loop repeats a block of code once for every item in a sequence. Each repetition is called an iteration. Here's the playlist idea in code:

playlist = ["Blinding Lights", "Levitating", "Shape of You"]

for song in playlist:
    print("Now playing:", song)

print("Playlist finished.")

Now playing: Blinding Lights
Now playing: Levitating
Now playing: Shape of You
Playlist finished.

The square brackets create a list: several values stored in order under one name. You'll study lists in depth in Python lists; for now, that's all you need to know.

The for loop has four parts:

• the keyword for,
• a loop variable (song) that holds the current item,
• the keyword in followed by the sequence to loop over (playlist),
• a colon and an indented loop body: the code that runs on every iteration.

The final print() isn't indented, so it runs only once, after the loop is done. Indentation works just like it does with if statements.

How does a for loop work step by step?

On each iteration, Python takes the next item from the sequence, stores it in the loop variable, and runs the body. When there are no items left, the loop ends and the program continues with the next unindented line.

You never have to tell the loop when to stop or move to the next item. Python handles that for you.

How do you loop through a string?

A string is a sequence of characters, so a for loop visits one character at a time:

for letter in "Hey!":
    print(letter)

H
e
y
!

Here's something more useful: counting the vowels in a word by combining a loop with an if statement.

word = "education"
vowels = 0

for char in word:
    if char in "aeiou":
        vowels += 1

print(f"'{word}' has {vowels} vowels.")

'education' has 5 vowels.

How do you repeat code a set number of times?

When you don't have a list and just want to repeat something, use range(). range(5) produces the numbers 0, 1, 2, 3 and 4: five numbers, starting at 0.

for i in range(3):
    print("Hip hip hooray!", i)

Hip hip hooray! 0
Hip hip hooray! 1
Hip hip hooray! 2

range() can also start at other numbers and count in steps. You'll learn all of that in the Python range() lesson.

How do you add up a total with a for loop?

A very common pattern is the accumulator: create a variable before the loop, then update it on every iteration. Here's a shopping cart total:

cart_prices = [12.99, 4.50, 29.00, 3.25]
total = 0

for price in cart_prices:
    total += price

print(f"Items: {len(cart_prices)}")
print(f"Total: ${total:.2f}")

Items: 4
Total: $49.74

The line total = 0 must come before the loop. If it were inside, the total would reset to 0 on every iteration. (len() gives the number of items in a list, the same way it gives the length of a string.)

How do you count items that match a condition?

Counting works the same way, but you only add 1 when a condition is true:

temperatures = [18, 25, 31, 22, 34, 29, 16]
hot_days = 0

for temp in temperatures:
    if temp >= 30:
        hot_days += 1

print(f"Hot days this week: {hot_days}")

Hot days this week: 2

Nested for loops

You can put a loop inside another loop. The inner loop runs completely for each iteration of the outer loop. A multiplication table is the classic example:

for row in range(1, 4):
    for col in range(1, 4):
        print(f"{row * col:4}", end="")
    print()

   1   2   3
   2   4   6
   3   6   9

range(1, 4) gives 1, 2 and 3. The :4 inside the f-string pads each number to 4 characters so the columns line up, end="" keeps the numbers on the same line (see print()), and the empty print() starts a new row. The inner loop runs 3 times for each of the 3 outer iterations, so the body runs 9 times in total.

Getting the position with enumerate()

Sometimes you need the item and its position. The built-in enumerate() gives you both on each iteration:

podium = ["Lewis", "Max", "Charles"]

for place, driver in enumerate(podium, start=1):
    print(f"{place}. {driver}")

1. Lewis
2. Max
3. Charles

Without start=1, counting begins at 0. You'll see enumerate() again in the built-in functions lesson.

Common mistakes with for loops

1. Forgetting the colon or the indent

for city in ["Paris", "Tokyo"]
    print(city)

SyntaxError: expected ':'

2. Resetting the accumulator inside the loop

for steps in [4000, 6500, 8200]:
    total = 0
    total += steps

print(total)

8200

Because total = 0 runs on every iteration, only the last number survives. Move it above the loop to get 18700.

3. Indenting a line that should run once

scores = [8, 9, 7]
total = 0
for s in scores:
    total += s
    print("Final total:", total)

Final total: 8
Final total: 17
Final total: 24

The summary line is inside the loop, so it prints three times. Un-indent it so it runs after the loop.

4. Trying to loop over a number

for i in 5:
    print(i)

TypeError: 'int' object is not iterable

A number isn't a sequence. To repeat 5 times, write for i in range(5):.

Practice exercise

runs = [5.2, 3.0, 7.5, 0, 4.8, 10.0, 0]
total = 0
rest_days = 0

for km in runs:
    total += km
    if km == 0:
        rest_days += 1

print(f"Total: {total} km")
print(f"Rest days: {rest_days}")

Total: 30.5 km
Rest days: 2

What's next?

A for loop is perfect when you know what you're looping over. But what if you want to keep going until something happens, like until the user types the right password? That's the job of the Python while loop.