#/usr/bin/env python3.2

class MyIterator():
    
    def __init__(self, step):
        self.step = step
        
    def __next__(self):
        """Returns the next element."""
        if self.step == 0:
            raise StopIteration
        self.step -= 1
        return self.step
    
    def __iter__(self):
        """Returns the iterator itself."""
        return self
    
for el in MyIterator(5):
    print(el, end=" ")  

console：

4 3 2 1 0