How to make a custom class work with the ‘for in’ loop in Python?

Question

If I have a custom class, how can I make it work with a ‘for in’ loop, like this:

class number_range:
    def __init__(self, start, end):
        self.__data = []
        for i in range(start, end):
            self.__data.append(i)
    #something that will allow the for in loop

numbers = number_range(1, 100)          
for number in numbers:
    print(number)

Answer 1

What you are looking for is an iterator. For example, this one acts like range but only gives you odd numbers:

class oddrange(object):
    def __init__(self, first, lastPlusOne):
        # Ensure start at first odd >= given start, store terminator.

        self.__curr = first + 1 - first % 2
        self.__term = lastPlusOne

    def __iter__(self):
        return self

    def __next__(self):
        # Terminate if range over, otherwise return current, calculate next.

        if self.__curr >= self.__term:
            raise StopIteration()

        (cur, self.__curr) = (self.__curr, self.__curr + 2)
        return cur

You can see this in action with the calling code:

for i in oddrange(-5, 9): print(i, end = ' ')
print()
for i in oddrange(-3, 9): print(i, end = ' ')
print()
for i in oddrange(-4, 10): print(i, end = ' ')
print()

This generates:

-5 -3 -1 1 3 5 7
-3 -1 1 3 5 7
-3 -1 1 3 5 7 9