for i in range(n - 1):

Is pretty much a standard for loop. Between that and

for n in numbers:

You can do pretty much the same things as a more conventional language.

You could also solve it pretty simply like this:

expected_sum = (n * (n + 1)) / 2

missing_num = expected_sum - sum(numbers)

This only iterates the list once. This would probably be my solution if I was given this task in an interview.