Why does python generator result like this?

Question

Here's the code:

def is_subsequence(a, b):
    b = iter(b)
    
    gen = ((i in b) for i in a)
    print(gen)

    for i in gen:
        print(i)

    return all(((i in b) for i in a))

res1 = is_subsequence([1, 3, 5], [1, 2, 3, 4, 5])
res2 = is_subsequence([1, 4, 3], [1, 2, 3, 4, 5])

The result is that res1==False and res2==False. Apparently the result for res1 is incorrect. When I comment out the for loop print, res1==True and res2==False, which is correct. I'm confused. Can someone explain why?

Answer 1

Generators are lazy iterators. You can loop over them like a list but once you do, they are exhausted, i.e. nothing is left. With the following line,

gen = ((i in b) for i in a)
print(gen)

you create a generator object and then with the following loop,

for i in gen:
    print(i)

you exhaust it. Incidentally you also exhaust b while exhausting gen. So by the time of the last line, b is empty, so your function will always return False.

IIUC, you want to evaluate if a is a subsequence (not subset) of b. So the order matters. You can change your function to:

def is_subsequence(a, b):
    b = iter(b)
    return all(((i in b) for i in a))

and it will work as intended.

Output:

res1 = is_subsequence([1, 3, 5], [1, 2, 3, 4, 5])  # True
res2 = is_subsequence([1, 4, 3], [1, 2, 3, 4, 5])  # False