Python Arithmetic Operators

Python Addition Operator (+)

If we want to add any two numbers, we use the + operator. Just like in maths.

print(5 + 3)      # 8
print(10 + 20)    # 30
print(1.5 + 2.5)  # 4.0

We can also add strings. This is called concatenation.

print("Hello" + " " + "World")   # Hello World

But we cannot add a string and a number.

print("Age: " + 25)   # Error!

Fix: We convert the number to a string first.

print("Age: " + str(25))   # Age: 25

Python Subtraction Operator (-)

If we want to subtract one number from another, we use the - operator. Same as maths.

print(10 - 3)     # 7
print(50 - 20)    # 30
print(1.5 - 0.5)  # 1.0

We can also subtract a negative number.

print(10 - (-3))   # 13

Because subtracting a negative is like adding.

Python Multiplication Operator (*)

If we want to multiply two numbers, we use the * operator. Same as maths.

print(5 * 3)      # 15
print(10 * 10)    # 100
print(2.5 * 4)    # 10.0

We can also multiply a string by a number. This repeats the string.

print("Ha" * 3)   # HaHaHa
print("-" * 20)   # --------------------

But we cannot multiply two strings.

print("Hello" * "World")   # Error!

Python Division Operator (/)

If we want to divide one number by another, we use the / operator. Same as maths.

print(10 / 2)     # 5.0
print(9 / 2)      # 4.5
print(10 / 3)     # 3.3333333333333335

Remember — division always gives a float. Always. Even if the answer is a whole number.

print(10 / 2)     # 5.0 (not 5)

That .0 at the end means it is a float.

Python Floor Division Operator (//)

Floor division divides and then rounds down to the nearest whole number.

print(10 // 2)    # 5
print(9 // 2)     # 4
print(10 // 3)    # 3

See? 9 // 2 is 4, not 4.5. It just chops off the decimal.

It does not round. It floors. Which means it goes to the lower number.

print(9 // 2)     # 4
print(-9 // 2)    # -5

Wait. Why is -9 // 2 equal to -5?

Because floor division rounds down. And -4.5 rounded down is -5.

That is a bit weird. But that is how it works.

Python Modulo Operator (%)

Modulo gives us the remainder after division.

print(10 % 3)     # 1

Why? Because 3 goes into 10 three times (9), and 1 is left over.

print(10 % 2)     # 0

Because 2 goes into 10 exactly five times. No remainder.

print(7 % 4)      # 3

Because 4 goes into 7 one time (4), and 3 is left over.

Modulo is very useful. For example, to check if a number is even or odd.

print(4 % 2)      # 0 (even)
print(5 % 2)      # 1 (odd)

If number % 2 == 0, it is even. Otherwise, odd.

Python Exponentiation Operator (**)

Raises a number to a power.

print(2 ** 3)     # 8 (2 * 2 * 2)
print(5 ** 2)     # 25 (5 * 5)
print(10 ** 0)    # 1

We can also use negative exponents.

print(2 ** -1)    # 0.5

Because 2 ** -1 is the same as 1 / 2.

Mistake 1: Confusing / and //

Here’s a mistake we all make. We write 10 / 3 thinking we will get 3. But we don’t. We get 3.3333333333333335.

print(10 / 3)     # 3.3333333333333335
print(10 // 3)    # 3

Why? Because / always gives us the full decimal answer. Always. If we want just the whole number part, we use //. That’s the only difference.

Mistake 2: Forgetting Order of Operations

Let’s say we write 2 + 3 * 4. What do we get? 14? Or 20?

We get 14. Not 20.

print(2 + 3 * 4)   # 14, not 20

Why? Because 3 * 4 happens first. That’s 12. Then 2 + 12 gives us 14.

If we want 20, we use parentheses.

print((2 + 3) * 4)   # 20

Now 2 + 3 happens first. That’s 5. Then 5 * 4 is 20.

Mistake 3: Trying to Do Math with Strings

Here’s one. We write "5" + 3. And we get an error.

print("5" + 3)   # Error!

Why? Because “5” is a string. And 3 is a number. Python doesn’t mix them.

So what do we do? We convert the string to a number first.

print(int("5") + 3)   # 8

Now it works. Because both are numbers.

Mistake 4: Thinking % is Percentage

We might think % means percentage. It doesn’t. It means modulo. It gives us the remainder.

print(10 % 3)   # 1 (remainder)

So 10 % 3 is 1. Not 10 percent of 3.

If we want percentage, we calculate it ourselves.

print(10 * 0.10)   # 1.0 (10% of 10)