C# and JS to python: Scoping in Python

development, python, csharp, and javascript

Problem

I have a function that should trigger a callback, lambda, Func, Action (or any other name that your language of choice might use for this concept). I also want to write a test to make sure that my function trigger this callback under certain conditions. The least invasive way I could think of to test this is to declare a variable in my test, and supply a callback that sets this variable when triggered.

Example code:

def __trigger_callback(callback: Callable[[], None]) -> None:
    callback()

def test_trigger():
    triggered = False

    def fetcher():
        triggered = True

    __trigger_callback(fetcher)

    assert triggered == True

As a JS and C# dev I would assume that this test would succeed as both of these languages will bind the variable name in the inner scope fetcher.triggered to the one defined in the outer scope test_trigger.triggered. This is not the case however, as python will create a new variable that is bound to the local scope, and the test will fail since the outer variable is never reassigned.

Solution

The solution to this is to use the nonlocal keyword, which will bind the local name to the outer variable.

def test_succeeds_because_of_nonlocal_rebinding():
    triggered = False

    def fetcher():
        nonlocal triggered;
        triggered = True

    __trigger_callback(fetcher)

    assert triggered == True

Usecase

I was building a self filling cache that would take a reference to a database fetching function, that it would trigger in the case of not finding the requested key in the cache.

Resources used when finding this solution

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