Operators and Expressions

Values on their own just sit there. Operators are the verbs that make them do something.

You can store numbers, text, and true-or-false values. Now it's time to work with them: add up scores, check whether someone passed, decide whether a student makes the honor roll. The symbols that do this work are called operators, and a piece of code that produces a value, like score + 5, is called an expression.

The calculator on your desk

Most of Python's math operators are the ones on a calculator, plus a few extras a calculator doesn't have. Python is often used as a calculator, in fact: open the interactive shell from How Python Runs, type 12 * 7, and press Enter.

Arithmetic operators

Operator	Meaning	Example	Result
+	add	7 + 2	9
-	subtract	7 - 2	5
*	multiply	7 * 2	14
/	divide	7 / 2	3.5
//	divide, whole part only	7 // 2	3
%	remainder	7 % 2	1
**	power	7 ** 2	49

The first four work just like math class. The last three deserve a closer look.

Two kinds of division

In Python, / always gives the exact answer, as a float, even when the numbers divide evenly:

print(7 / 2)
print(10 / 2)

3.5
5.0

Sometimes you want only the whole part. How many full boxes of 6 eggs can you fill with 20 eggs? That's floor division, //:

print(20 // 6)

3

And how many eggs are left over? That's the remainder, %:

print(20 % 6)

2

// and % are a pair: 20 eggs make 3 full boxes with 2 left over. They're more useful than they look:

• Even or odd? A number is even when number % 2 is 0.
• Converting units. 135 minutes is 135 // 60 hours (2) and 135 % 60 minutes (15).
• Taking turns. With 4 players, turn number n belongs to player n % 4.

If you've taken the Java track, notice the difference: in Java, 7 / 2 is 3, because dividing two whole numbers drops the fraction. Python's / never does that. Use // when you want it.

Powers: **

** raises a number to a power. 2 ** 3 means 2 × 2 × 2:

print(2 ** 3)
print(10 ** 6)

8
1000000

Order of operations

Python follows the order you learned in math class. ** comes first, then *, /, //, and %, then + and -. Operators at the same level work left to right:

print(2 + 3 * 4)
print((2 + 3) * 4)

14
20

When in doubt, add parentheses. They cost nothing and make your meaning obvious.

Shortcuts: compound assignment

Updating a variable using its own value is so common that Python has shorthand for it. You met two of these in Variables and Naming:

Shorthand	Means
score += 5	score = score + 5
score -= 5	score = score - 5
score *= 2	score = score * 2
score /= 2	score = score / 2
score //= 2	score = score // 2
score %= 2	score = score % 2

Python doesn't have ++ or --. Where other languages write count++, Python writes count += 1.

Comparison operators

Comparison operators ask a question about two values and answer with a bool: True or False.

Operator	Asks	Example	Result
==	equal to?	5 == 5	True
!=	not equal to?	5 != 3	True
>	greater than?	5 > 3	True
<	less than?	5 < 3	False
>=	greater than or equal?	5 >= 5	True
<=	less than or equal?	4 <= 3	False

You can store the answer in a variable:

score = 82
passed = score >= 75
print(passed)

True

Watch the difference between = and ==. One equals sign stores a value. Two equals signs compare values. Python catches the most common mix-up for you:

score = 82
if score = 100:  # error: SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
    print("Perfect!")

Python even suggests the fix. (:= is a more advanced operator you won't need in this track.)

Comparisons work on text too. == checks whether two strings are exactly the same, capital letters included:

print("Manila" == "Manila")
print("Manila" == "manila")

True
False

Chained comparisons

Here's something Python does that most languages don't. To check whether a number is in a range, you can write it just like in math:

age = 15
print(13 <= age <= 19)

True

Python reads 13 <= age <= 19 as "13 is at most age, and age is at most 19." In Java or JavaScript, the same line either fails or gives a wrong answer. In Python, it's the clearest way to write a range check.

Logical operators

Sometimes one question isn't enough. "Is the score at least 90 and is attendance good?" Python's logical operators are plain English words:

Operator	True when
and	both sides are true
or	at least one side is true
not	flips True to False, and False to True

score = 93
absences = 2

honor_roll = score >= 90 and absences <= 3
needs_help = score < 75 or absences > 10
is_failing = not score >= 75

print(honor_roll, needs_help, is_failing)

True False False

A handy way to remember them: and is strict (everything must be true), or is generous (anything true will do), and not is contrary (it says the opposite).

Python is also a bit lazy with and and or, in a helpful way. If the left side of and is False, the whole thing must be false, so Python never even looks at the right side. Likewise, if the left side of or is True, Python stops there. This is called short-circuiting, and it will save you from a crash in the Try it section below.

Try it

A student has three quiz scores. This program works out the total, the average, whether the total is even, and whether the student makes the honor roll. Predict every line before you check.

Running Python here

Running Python right in the browser is coming to Syntaxia soon. For now, you can read and edit the code here. If you have Python installed on your computer (see Setting Up), save it as main.py and run it with python main.py (or python3 main.py on macOS and Linux).

Check your prediction

Total: 265
Average: 88.33333333333333
Whole-number average: 88
Points above 250: 15
Is the total even? False
Honor roll? True
In the B range? True
Can split points into teams? False

total / 3 gives the exact average as a float, while total // 3 keeps only the whole part. On the last line, teams != 0 is False, so and stops right there and never tries total / teams. Without short-circuiting, dividing by zero would crash the program.

Try it yourself

1. Change quiz3 so the honor roll line prints False. What's the highest score that does it?
2. Add a line that turns 135 minutes into hours and minutes using // and %, and prints 2 hours and 15 minutes.
3. Swap the two sides of the and on the can_split line, so it reads total / teams > 10 and teams != 0. What would happen now, and why?

Check your understanding

What does print(7 // 2) print?

3.5

3

4

1

What is 20 % 6?

3

3.33

14

2

Which line checks that age is between 13 and 19, inclusive, in Python?

13 <= age <= 19

13 < age < 19

age between 13 and 19

13 <= age or age <= 19

Up next

You saw that Python refuses to add a number to a string. So how do you turn the number 17 into the text "17", or the text a user typed into a number you can do math with? That's Type Conversion.