Closure Late Binding in a Loop

Problem

An HTTP framework registers one handler per route. To avoid repeating the dispatch boilerplate, the handlers are created in a loop:

def dispatch(route: str, request: str) -> str:
    return f"{route} handled {request}"


def build_handlers(routes: list[str]) -> dict[str, "Callable[[str], str]"]:
    handlers = {}
    for route in routes:
        handlers[route] = lambda request: dispatch(route, request)
    return handlers


handlers = build_handlers(["/users", "/orders", "/health"])
print(handlers["/users"]("GET"))

Symptom: every route is handled as if it were /health. The line above prints /health handled GET. The dictionary keys are correct (/users, /orders, /health), only the behaviour is wrong. Explain what is printed and why, and fix build_handlers.

Examples

• handlers["/users"]("GET") → expected /users handled GET. Actual /health handled GET.
• handlers["/orders"]("POST") → expected /orders handled POST. Actual /health handled POST.
• handlers["/health"]("GET") → /health handled GET. Correct only because it happens to be the last route.

Hints

1. When is the name route inside the lambda looked up: when the lambda is created, or when it is called?
2. How many distinct variables named route exist in build_handlers? One per iteration, or one for the whole loop?
3. Give each lambda its own copy of the value at creation time. Default arguments are evaluated at definition time.