Type Casting and Type Conversion in Python

Data often arrives in the wrong type: numbers typed by a user are strings, values from a CSV file are strings, and sometimes you need a list from a string or a string from a number. Type conversion changes a value from one type to another.

Python performs some conversions automatically (implicit), and you perform others with functions like int(), float(), str(), list() and bool() (explicit, also called type casting). This lesson covers both, plus what happens when conversion fails.

Implicit Conversion

Python automatically widens numeric types in mixed arithmetic: int + float produces a float and int + complex a complex, so no precision is lost. It never silently converts between strings and numbers — "5" + 3 raises TypeError.

Explicit Conversion Functions

The main casting functions:

• int(x) — from float (truncates toward zero), from string of digits, or with a base: int("ff", 16).
• float(x) — from int or numeric string, including "1e3", "inf", "nan".
• str(x) — a readable string of any object.
• bool(x) — truthiness of any value.
• list(x), tuple(x), set(x), dict(pairs) — between collection types.
• chr(n) / ord(c) — between characters and Unicode code points; bin(), oct(), hex() — integers to base strings.

When Conversion Fails

int("12.5") and int("abc") raise ValueError; int(None) raises TypeError. Wrap conversions of external data in try/except. To turn "12.5" into an int, convert to float first. Note that bool("False") is True because the string is not empty.

Examples

result = 10 + 2.5
print(result, type(result))

print(int(9.99), int(-9.99), int("42"), int("ff", 16), int("1010", 2))
print(float(7), float("3.5"), float("1e3"))
print(str(3.14) + " is pi")
print(int(float("12.5")))

12.5 <class 'float'>
9 -9 42 255 10
7.0 3.5 1000.0
3.14 is pi
12

print(list("abc"), tuple([1, 2]), set([1, 1, 2]))
print(dict([("a", 1), ("b", 2)]))
print(ord("A"), chr(97), chr(8377))
print(bin(10), oct(10), hex(255))
print(bool("False"), bool(""), bool(0.0))

['a', 'b', 'c'] (1, 2) {1, 2}
{'a': 1, 'b': 2}
65 a ₹
0b1010 0o12 0xff
True False False

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

for raw in ["25", " 7 ", "12.5", "abc", None]:
    print(repr(raw), "->", to_int(raw))

try:
    print("5" + 3)
except TypeError as error:
    print("TypeError:", error)

'25' -> 25
' 7 ' -> 7
'12.5' -> 0
'abc' -> 0
None -> 0
TypeError: can only concatenate str (not "int") to str