Functions

Defining a function

def add(a: int, b: int) -> int:
    return a + b

def main() -> None:
    print(add(1, 2))

All parameters and return types must be annotated. Functions returning nothing use -> None.

Overloads

Multiple functions with the same name but different parameter types are supported:

import math

def twice(x: int) -> int:
    return x + x

def twice(x: float) -> float:
    return x + x

def main() -> None:
    print(math.abs(-5))
    print(math.abs(-3.5))
    print(twice(4))
    print(twice(1.25))

Overload resolution rules

• Overloads are resolved at compile time from parameter types only.
• Exact match wins.
• int → float widening is allowed when no exact match exists.
• Ambiguous matches (two equally-valid candidates) are rejected with a diagnostic that lists all candidates.
• Return type does not participate in resolution.
• Methods are not overloaded.

Return flow

Non-None functions must return on all paths. py4 checks this statically:

def classify(x: int) -> str:
    if x > 5:
        return "big"
    elif x == 5:
        return "mid"
    else:
        return "small"

def main() -> None:
    print(classify(3))
    print(classify(5))
    print(classify(8))

Top-level only

Functions must be declared at module scope. Nested functions and lambdas are not supported.