access-variables-outside-loop-scope.md

Access Variables Outside Loop Scope

Here is a function that loops over a list to find the first occurrence of a falsy value.

def find_false(self):
    for item in self.items:
        item_type = type(item)
        print(f"Current item: {item} ({item_type})")
        if not item:
            break

    print(f"First false item: {item} ({item_type})")

Notice how at the end of the function, outside of the loop, I am able to access both item (defined in the loop definition) and item_type (defined within the loop's body).

Both of these variables are defined, by the loop, in function scope and are accessible anywhere in the function after they have been defined.

The title of this TIL is a bit of a misnomer because Python doesn't have the concept of a loop scope. There are two levels of scope in Python -- module/global scope and function scope.

I spend most of my time writing Ruby which also has block scope, so Python's simplified two-level scoping took me by surprise.

Though the code sample above is contrived, this function scope assignment can be taken advantage of with loop definitions in scenarios where you want to know what the last item defined was before the loop terminated.

for submission in submissions:
    if passes(submission, criteria):
        break
else:
    raise ValueError("No submissions that meet given criteria")

print(f"Submit first passing submission: {submission.id}")
submit(submission)