局部变量“result”可能在赋值前被引用

Question

With a flow like this:

def func():
    try:
        result = calculate()
    finally:
        try:
            cleanup()
        except Exception:
            pass
    return result

There is a warning about Local variable 'result' might be referenced before assignment :

But I can't really see how. One of these must be true:

• calculate() raises an exception -> the return statement will never get reached
• calculate() does not raise an exception -> result returns a value

How would you ever get result referenced before assignment?

Answer 1

Since there is no except (catch) block the detector assumes that the try block can fail, so the result will never be assigned.
Moving forward, the finally block will be executed, and here cleanup() can success, so since there is no return for now, the detector assumes that the result can be reached without having an assigned value.

To be more explicit, this is the flow that can reach result without assigning any value to it:

First try fails -> It goes to finally block -> here the cleanup() works and do not interupt anything -> It goes further and returns result (which wasn't initialized yet).

Even if you are sure that this cannot happen, the detector is warning you about this posibility.