GCSE Computer Science

Python type casting — GCSE Computer Science

The most common casting pattern — converting input() to a number — is covered in the Python variables guide. This page covers casting as its own topic: all four conversion functions, casting between numeric types, and what happens when casting fails.

The four casting functions

• int() — converts to a whole number.
• float() — converts to a decimal number.
• str() — converts to text.
• bool() — converts to True or False.

Casting between numeric types

pi = 3.9
print(int(pi))     # 3
print(float(7))    # 7.0

int() doesn't round — it truncates, cutting off everything after the decimal point and keeping only the whole-number part. int(3.9) is 3, not 4, and int(-3.9) is -3, not -4. Rounding and truncating are different operations, and exams test the difference deliberately.

Why casting fails

age = int("hello")

This raises a ValueError — not because the syntax is wrong, but because "hello" isn't a valid whole number, and Python can't turn it into one. Casting only works when the value you're converting actually represents the target type. int("42") works; int("forty-two") doesn't.

str() for combining numbers with text

score = 85
message = "You scored " + str(score)
print(message)

Python won't let you concatenate a string and a number directly with + — str() converts the number to text first so the concatenation works.

bool() — truthy and falsy values

print(bool(0))      # False
print(bool(1))      # True
print(bool(""))     # False
print(bool("hi"))   # True

0, an empty string "", and None all convert to False. Almost everything else converts to True. This matters when a value is used directly as a condition — if my_list: is really asking bool(my_list), which is False only when the list is empty.

What examiners actually test

Casting questions usually ask you to state the output of code involving int(), float(), or str(), identify which cast is needed to fix a type error, or explain why a given cast would fail. The truncation-not-rounding rule for int() is the single most commonly tested casting fact — know it as a fixed rule, not a guess.