closure-example.py

gistfile1.txt

# from Fluent Python 2nd ed

class Averager():

    def __init__(self):
        self.series = []

    def __call__(self, new_value):
        self.series.append(new_value)
        total = sum(self.series)
        return total/len(self.series)


def make_averager():
    # closure, variable still available after local scope gone
    # a function "closes over" any objects it maintains a reference to those objects
    # https://stackoverflow.com/a/13902

    count = 0 # immutable type > oops!
    series = [] # mutable type no issue

    def averager(new_value):
        # without using nonlocal on count, you'd get an exception here:
        # UnboundLocalError: local variable referenced before assignment
        # because you try to assign a value to a local variable before it has been declared.
        nonlocal count
        count += 1
        series.append(new_value)
        total = sum(series)
        return total/len(series)

    return averager


if __name__ == '__main__':
    avg = Averager()
    print(avg(10), avg(11), avg(12))

    avg2 = make_averager()
    print(avg2(10), avg2(11), avg2(12))