Python generator on dictionary view does not seem to be lazily evaluated

Question

It's my understanding that python generator expressions return lazily evaluated comprehensions, and view objects on dictionaries update when their underlying dictionary is changed.

Given the following code (Python 2.7.6 Windows):

d = {}
gt_three = (x for x in d.viewkeys() if x > 3)
print list(gt_three)
d[1] = 'one'
d[4] = 'four'
print list(gt_three)

I would expect output like:

[]
[4]

Instead, I'm receiving:

[]
[]

If I recreate the generator after adding to the dictionary, or print the view itself, I do see the expected output. What am I not understanding about generators? I'm quite new to python.

Answer 1

A generator expression returns an iterator, which you can only iterate over once. Attempts to iterate over an iterator after the first time will always see it as empty.