What functions should an object implement to be traversed using the 'in' loop in Python?

For an object to be iterable with a for ... in loop in Python, it must implement the iteration protocol. This means the object must have an __iter__() method that returns an iterator.

The iterator, in turn, must implement the __next__() method, which returns the next element in the sequence and raises a StopIteration exception when the sequence ends.

Example:

class MyIterable:
    def __init__(self, data):
        self.data = data
    def __iter__(self):
        self.index = 0
        return self
    def __next__(self):
        if self.index < len(self.data):
            result = self.data[self.index]
            self.index += 1
            return result
        else:
            raise StopIteration

obj = MyIterable([1, 2, 3])
for item in obj:
    print(item)

Thus, having __iter__() and __next__() methods allows an object to be iterable.