kmarekspartz

On (not) reusing generators in Python

A friend of mine had an issue recently with a generator in Python. Here's the code:

def our_generator(input):
    while input:
        do(something, on, input)
        yield something

print(list(our_generator(input)))

for i in our_generator(input):
    do(something, else, with, i)

See the bug?

There was an issue with the for-loop, so the print statement was added in, but this changed the behavior of the program. Once the generator has expired, it can't be reused, so it never entered the for-loop. Since the generator would be exhausted anyway, we can get the expected behavior without additional cost by doing the following:

output = list(our_generator(input))

print(output)

for i in output:
    do(something, else, with, i)