8.4 Numeric Type Conversion

The numeric types themselves are covered in 5.3 Numeric Data Types — this section focuses specifically on the rules for converting into each one.

int()

Converts a string of digits, or truncates a float toward zero (discarding the fractional part — it does not round). Also converts bool (True → 1, False → 0).

>>> int("42")
42
>>> int(3.9)      # truncates -- NOT rounded
3
>>> int(True)
1

float()

Converts a numeric string or an int to a floating-point value.

>>> float("3.14")
3.14
>>> float(5)
5.0

complex()

Converts a number (or a string in a+bj form) into a complex number with real and imaginary parts.

>>> complex(2)
(2+0j)
>>> complex("3+4j")
(3+4j)

bool()

Converts any value to True or False using Python’s truthy/falsy rules — see 8.8 Boolean Conversion and Type Checking.

>>> bool(0), bool(1), bool(""), bool("x")
(False, True, False, True)

Conversion Rules

int() discards the .99 fractional part — it does not round. Use int(round(3.99)) → 4 if rounding is what’s actually needed.

The most important rule to internalize: int(float_value) always rounds toward zero, never to the nearest whole number. Use round() first if rounding is actually the goal:

>>> int(3.9), int(-3.9)      # both truncate toward zero
(3, -3)
>>> int(round(3.9))           # round first, then convert
4

Quick Interview Answer

“int() parses a digit string or truncates a float toward zero — critically, it does not round, so int(3.9) is 3, and int(-3.9) is -3, not -4. float() parses a numeric string or widens an int. complex() builds a complex number from a real number or an a+bj string. bool() applies the truthy/falsy rules explicitly. When rounding to the nearest whole number is actually the goal, round() first and int() second: int(round(3.9)) is 4.”

Common Mistakes

• Expecting int(3.9) to round to 4 — it truncates toward zero, giving 3; use round() first for actual rounding.
• Forgetting int(-3.9) truncates to -3 (toward zero), not -4 (toward negative infinity, which is what // does) — the two are easy to conflate.
• Trying int("3.14") directly — it raises ValueError, since int() expects a string that’s already a whole number; go through float() first (see 8.15 Common Mistakes).