Variables & core types

Variables: names for values

A variable is a name you attach to a value so you can use it later:

price = 1500
quantity = 3
print(price * quantity)

4500

The = means "store the right side under the name on the left". Read price = 1500 as "price gets 1500", not "price equals 1500".

Naming rules

• Letters, digits, underscores — but cannot start with a digit: item2 is fine, 2item is not.
• Case matters: Price and price are different variables.
• Style: lowercase with underscores — total_cost, not TotalCost or tc. Name things so future-you understands them.

The four core types

age = 25              # int    — whole numbers
price = 499.99        # float  — decimal numbers
name = "Chidinma"     # str    — text, in quotes
is_paid = True        # bool   — True or False (capitalised!)

Ask Python what type something is with type():

>>> type(1500)
<class 'int'>
>>> type("1500")
<class 'str'>

Notice: 1500 and "1500" are different things. One is a number you can do maths with; the other is text that happens to look like a number.

Arithmetic

print(10 + 3)    # 13
print(10 - 3)    # 7
print(10 * 3)    # 30
print(10 / 3)    # 3.3333333333333335  (/ always gives a float)
print(10 // 3)   # 3   (floor division — whole part only)
print(10 % 3)    # 1   (remainder / modulo)
print(10 ** 3)   # 1000 (power)

When types collide: your first traceback

Type this and run it:

price = "1500"
total = price * 3 + 100

Traceback (most recent call last):
  File "shop.py", line 2, in <module>
    total = price * 3 + 100
            ~~~~~~~~~~^~~~~
TypeError: can only concatenate str (not "int") to str

Do not panic — read the error first, and read it bottom-up. The last line names the problem: TypeError — you tried to add an int to a str. The line above it shows exactly where. ("1500" * 3 is "150015001500" — Python repeats strings — and then adding 100 to text fails.)

This bottom-up habit is the most valuable debugging skill in this course. The last line says what went wrong; the lines above say where.

Try it now

Create budget.py. Store your monthly data bundle cost, transport cost, and food budget in three well-named variables. Print the total and print what type each variable is using type(). Then deliberately put quotes around one of the numbers, run it, and read the traceback bottom-up before fixing it.