lazylen.py

class LazyLength(object):
    def __init__(self, data):
        self.data = data
    def __iter__(self):
        len = 0
        for x in self.data:
            yield x
            len += 1
        self.len = len
    def __len__(self):
        if hasattr(self, 'len'):
            return self.len
        raise ValueError("IDK")

l = LazyLength(range(10))
len(l)
### Traceback (most recent call last):
###   File "<input>", line 1, in <module>
###   File "<input>", line 13, in __len__
### ValueError: IDK

for i in l:
    print i
    

### 0
### 1
### 2
### 3
### 4
### 5
### 6
### 7
### 8
### 9
len(l)
### 10
###