TypeGuard and Type Narrowing

TypeGuard helps type checkers narrow down types within function bodies, improving type safety.

Type Narrowing with Isinstance

Use isinstance to narrow types within conditional blocks.

from typing import Union

def process_value(value: Union[int, str]) -> str:
    if isinstance(value, int):
        # Type checker knows value is int here
        return f"Integer: {value * 2}"
    else:
        # Type checker knows value is str here
        return f"String: {value.upper()}"

print(process_value(5))
print(process_value("hello"))

Integer: 10
String: HELLO

TypeGuard for Custom Type Checks

Create custom type guards for more complex type narrowing.

from typing import TypeGuard

def is_list_of_ints(value: list) -> TypeGuard[list[int]]:
    return all(isinstance(item, int) for item in value)

def process_numbers(value: list) -> int:
    if is_list_of_ints(value):
        # Type checker narrows to list[int]
        return sum(value)
    else:
        return 0

print(process_numbers([1, 2, 3]))
print(process_numbers([1, "two", 3]))

6
0