LazyList

gistfile1.py

class LazyList():
    def __init__(self, iterable):
        self._list = []
        self._iterable = iter(iterable)
        
        self._has_thrown = False
        self._current_index = 0
        
    def __iter__(self):
        return self
        
    def _cached_next(self):
        if self._current_index >= len(self._list):
            raise StopIteration()
        else:
            index = self._current_index
            self._current_index += 1
            return self._list[index]
    
    def _uncached_next(self):
        _next = self._iterable.next()
        self._list.append(_next)
        return _next
    
    def next(self):
        try:
            return self._cached_next() if self._has_thrown else self._uncached_next()
        except StopIteration:
            self._has_thrown = True
            self._current_index = 0
            
            raise StopIteration()
        


y = LazyList((a*a for a in [1, 2, 3, 4]))
# y = LazyList([1, 4, 9, 16])

for x in y:
    print x**2
    
for x in y:
    print x**3
    
for x in y:
    print x**4