Python — Casting

Type conversion functions

Python converts types explicitly with constructor functions:

x = int(3.99)      # 3     — truncates, never rounds!
y = float(5)       # 5.0
z = str(42)        # "42"
b = bool("hi")     # True

int() — the strict one

int("42")       # 42   ✓ digits only
int("3.5")      # ValueError! strings must be whole numbers
int(3.99)       # 3    floats truncate toward zero
int(-3.99)      # -3   (not -4)

Need a decimal string as int? Two-step:

int(float("3.5"))   # 3

input() always returns strings

The most important casting fact for beginners:

age = input("Age? ")     # user types 25
print(age + 1)           # TypeError: can't concat str to int

age = int(input("Age? "))
print(age + 1)           # 26 ✓

Any math on input() results requires conversion first.

str() — for mixing with text

age = 25
print("I am " + str(age))          # works
print(f"I am {age}")               # f-strings convert FOR you — prefer this

float()

float("3.14")    # 3.14
float(7)         # 7.0
float("abc")     # ValueError

bool() — the truthiness machine

bool(0)         # False
bool("")        # False
bool([])        # False
bool("False")   # True!! non-empty string
bool(None)      # False

Falsy values: 0, 0.0, "", [], {}, None, False. Everything else is truthy.

Safe conversion pattern

Wrap risky casts in try/except:

def to_int(text, fallback=0):
    try:
        return int(text)
    except ValueError:
        return fallback

to_int("42")     # 42
to_int("abc")    # 0 — controlled failure instead of a crash

Checking types

type(x) is int          # exact type
isinstance(x, (int, float))   # accepts subclasses/multiple — preferred

Gotchas: int() truncates rather than rounds (int(2.7) → 2; use round(2.7) → 3) · bool("False") is True · implicit coercion basically doesn't exist in Python ("1" + 1 raises, unlike JS).

Mini Practice

1. Convert "3.7" to an int two ways.
2. Build an age-asker that survives non-numeric input.
3. Test bool() on ten values; predict before running.
4. Show str(4) + "4" vs 4 + 4.
5. Write is_number(text) returning True/False using try/except.

Next: booleans →