Python Casting

Definition: Casting (or type conversion) means changing a value from one type to another — for example turning the text "10" into the number 10.

The conversion functions

• int(x) — convert to a whole number
• float(x) — convert to a decimal number
• str(x) — convert to text

Example 1 — text to number and back

a = int("10")     # "10" -> 10
b = float("3.5")  # "3.5" -> 3.5
c = str(99)       # 99 -> "99"
print(a + 5)
print(b)
print(c + "!")

Example 2 — why it is needed

Numbers typed by a user arrive as text. You must cast before doing maths:

price = "20"
qty = "3"
total = int(price) * int(qty)
print("Total:", total)

Without int(), "20" * "3" would error.

Example 3 — number to text for joining

You cannot join text and a number with +, so convert the number first:

age = 25
print("I am " + str(age) + " years old")

💡 Watch out: int("hello") fails — you can only convert text that actually looks like a number.