Variables & Numbers

A variable is a name pointing at a value. You create one by assigning to it — no keyword, no type declaration, no ceremony.

bill = 40
tip_rate = 0.18

tip = bill * tip_rate
print(tip)
print(bill + tip)

Read = as “gets”, not as “equals”: bill = 40 means the name bill now refers to 40. That is why count = count + 1 makes sense, which as a mathematical claim would be nonsense.

Naming things

Python names use lower_snake_case. They may contain letters, digits and underscores, and may not start with a digit.

total_price = 19.99      # good
totalPrice = 19.99       # works, but not the Python style
total price = 19.99      # SyntaxError - no spaces
2nd_price = 19.99        # SyntaxError - cannot start with a digit

Names are worth thinking about for a moment. n, data and temp tell a reader nothing; unread_count, sales_rows and celsius tell them everything. You will read your own code far more often than you write it.

The number types

Python has two everyday number types: int for whole numbers and float for decimals. type() tells you which you have.

print(type(7))
print(type(7.0))
print(type(7 + 0.5))

# Dividing always produces a float, even when it divides evenly.
print(10 / 2, type(10 / 2))

Notice the last line: 10 / 2 is 5.0, not 5. Plain / is true division and always returns a float.

Arithmetic operators

// and % are the pair you reach for when you need whole units and a leftover — minutes and seconds, pages and rows, coins and change.

total_seconds = 227

minutes = total_seconds // 60
seconds = total_seconds % 60

print(minutes, "minutes and", seconds, "seconds")

# % is also the standard "is this divisible?" test:
print(10 % 2, 11 % 2)

Operator precedence follows normal maths: ** first, then * / // %, then + -. When in doubt, use parentheses — they cost nothing and save the reader a moment of doubt.

Updating a variable

x = x + 1 works, but Python offers a shorthand for every operator:

score = 10
score += 5      # same as score = score + 5
score -= 3
score *= 2
print(score)

message = "Hi"
message += " there"   # works on strings too
print(message)

print(0.1 + 0.2)
print(0.1 + 0.2 == 0.3)

# For money, count in the smallest unit - cents - using ints.
price_cents = 1999
quantity = 3
print("Total: $", (price_cents * quantity) / 100)

# Or round when displaying.
print(round(0.1 + 0.2, 2))

The rule of thumb: use int for anything counted, float for anything measured, and never compare two floats with ==.

Before you write your own

Predict. What does each print show? Decide all three lines first.

coffees = 3
price = 4
coffees += 2
print(coffees * price)
print(17 // 5, 17 % 5)
print(8 / 4)

20
3 2
2.0

Fix. This should say how many whole weeks and leftover days are in 45 days — 6 weeks and 3 days — but it doesn’t. Run it, see what it does print, and fix the two operators.

days = 45
weeks = days / 7
leftover = days // 7
print(weeks, "weeks and", leftover, "days")

days = 45
weeks = days // 7
leftover = days % 7
print(weeks, "weeks and", leftover, "days")

6 weeks and 3 days

Total with tip: 162.25
Each person pays: 54.08

bill = 137.50
people = 3
tip_rate = 0.18

# Your code here

bill = 137.50
people = 3
tip_rate = 0.18

total = bill * (1 + tip_rate)
each = total / people

print("Total with tip:", round(total, 2))
print("Each person pays:", round(each, 2))

What you learned

• name = value binds a name to a value; no declaration needed.
• int counts, float measures, and type() tells you which you have.
• / always gives a float; // and % give whole units and leftovers.
• += and friends update a variable in place.
• Float arithmetic is approximate — round for display, use ints for money.