handlers = []

def register(func, data_type):
    handlers.append((func, data_type))


def handle(data):
    for func, data_type in handlers:
        d = data.get(func, data_type())
        data[func] = func(d)


def counter(x):
    print('counter', x)
    return x + 1


register(counter, int)
register(lambda s: s + 'a', str)

data = {}
handle(data)
handle(data)
handle(data)
print(data)