Python Assignment Operators

Programs constantly update values: a game adds points, a bank account loses money when you pay, a fitness tracker counts every step. Python assignment operators are the tools for storing and updating values like these. In this lesson, you'll learn the plain =, the handy shortcut operators like += and -=, and the newer "walrus" operator :=.

What is an assignment operator in Python?

An assignment operator puts a value into a variable. The simplest one is =, which you've used since Python variables. Python first works out everything on the right side, then stores the result in the name on the left.

price = 20
quantity = 3
total = price * quantity   # right side first: 20 * 3, then store 60
print(total)

60

Very often, though, the new value depends on the old one. You could write the variable name twice:

score = 0
score = score + 10
score = score + 25
print(score)

35

This works, but Python gives you a shorter way.

What does += mean in Python?

+= means "add to the current value and store the result back". So score += 10 does exactly the same as score = score + 10. Operators like this are called compound assignment operators, because they combine a math operation with an assignment.

score = 0
score += 10   # collected a coin
score += 25   # beat a level
print(score)

35

Every arithmetic operator has a matching compound version:

How compound assignment works step by step

Let's follow a bank balance through a few transactions. Each line reads the current value, does the math, and stores the new value under the same name:

balance = 100
balance += 50    # payday bonus
print(balance)

balance -= 30    # groceries
print(balance)

balance *= 2     # a generous relative doubles it
print(balance)

150
120
240

Using -=, *= and /= in real programs

Here's a shopping cart where the subtotal grows, a discount is applied, and the result is shared between two people:

cart_total = 0
cart_total += 35    # running shoes
cart_total += 15    # socks
cart_total *= 0.9   # 10% off
print(cart_total)

cart_total /= 2     # split with a friend
print(cart_total)

45.0
22.5

Notice that cart_total became a float after *= 0.9. Compound operators follow the same type rules as the normal arithmetic operators, so /= always turns the variable into a float, even if it was an int.

pizzas = 8
pizzas /= 2
print(pizzas)

slices = 8
slices //= 2
print(slices)

4.0
4

Using //=, %= and **=

These three are less common, but each has its moment. %= is great for wrap-around values, like a playlist that goes back to the first song after the last one:

song_count = 5
current_song = 4      # the last song (songs are numbered 0 to 4)

current_song += 1
current_song %= song_count
print(current_song)

0

And **= is useful for anything that grows by multiplying, like a social post being shared:

viewers = 3
viewers **= 2    # each viewer shares it with 3 more people
print(viewers)

seconds = 200
seconds //= 60   # whole minutes only
print(seconds)

9
3

Do compound operators work with strings?

Yes. += joins text onto the end of a string, and *= repeats it. This is handy for building a message bit by bit:

order = "Latte"
order += ", oat milk"
order += ", extra shot"
print(order)

cheer = "Go! "
cheer *= 3
print(cheer)

Latte, oat milk, extra shot
Go! Go! Go! 

What is the walrus operator (:=) in Python?

The walrus operator := (named because := looks like a walrus's eyes and tusks) assigns a value and gives that value back in the same step. That lets you assign inside a bigger expression, such as a condition. It was added in Python 3.8. Here's a preview using an if statement, which you'll learn in if...else:

budget = 50
price = 38

if (change := budget - price) >= 10:
    print("You have", change, "left. Grab a dessert!")

You have 12 left. Grab a dessert!

Without the walrus, you'd calculate change on one line and check it on the next. As a beginner, you rarely need :=; the regular = is clearer. Just recognize it when you see it.

Real-world example: a step counter

daily_goal = 10000
steps = 0

steps += 3200   # walk to work
steps += 1850   # lunch break
steps += 4100   # evening jog

remaining = daily_goal
remaining -= steps
print("Steps today:", steps)
print("Steps to goal:", remaining)

Steps today: 9150
Steps to goal: 850

Common mistakes with assignment operators

1. Updating a variable that doesn't exist yet

steps += 100

NameError: name 'steps' is not defined

steps += 100 needs a current value to add to. Create the variable first, for example steps = 0.

2. Writing =+ instead of +=

lives = 3
lives =+ 1
print(lives)

1

No error, but the wrong answer! Python reads lives =+ 1 as lives = +1, which simply stores positive one. The math symbol always comes before the equals sign.

3. Trying to use ++

count = 0
count++

SyntaxError: invalid syntax

Use count += 1 instead.

4. Confusing = with ==

= stores a value. == asks "are these equal?" and gives True or False. You'll learn == in the next lesson.

Practice exercise

coins = 50
coins += 30   # treasure chest
coins -= 45   # sword
coins *= 2    # power-up
print("Coins:", coins)

Coins: 70

What's next?

You can now store and update values. Next, learn how to compare values and get True or False answers with Python comparison operators.