def gen_yy():
    yield 1
    yield 2

def gen_yr():
    yield 1
    return 2

print('Correctly prints 1 and 2')
for i in gen_yy():
    print(i)
print('-'*15)
print()

print('Only prints 1, skips printing 2')
for i in gen_yr():
    print(i)
print('-'*15)
print()

print('Actual use case for returning from a generator')
def external_function(r):
    return r < 0.10

from random import random
def work_until_something_goes_wrong():
    while True:
        r = random()
        if external_function(r):
            return -1
        else:
            yield 0

print('This will never print the -1 returned as StopIteration(-1)')
for result in work_until_something_goes_wrong():
    print(result)
print('-'*15)
print()
# it's too late to retrieve the -1 from the generator because it's reference is lost
# the -1 was raised to the for-loop as StopIteration(-1) however the loop body doesn't execute the final iteration so -1 isn't printed