LazyList in python

class LazyList:
    def __init__(self, it, cache_step=4):
        self.it = iter(it)
        self.cache = []
        self.cache_step = cache_step

    def __getitem__(self, i):
        while True:
            try:
                return self.cache[i]
            except IndexError:
                pre = len(self.cache)
                self.cache.extend(x for _, x in zip(range(self.cache_step - 1), self.it))
                if pre == len(self.cache):
                    raise IndexError