使用其他功能更改生成器-Python 3.x

Question

I want to change the behavior of the generator below so that it only yields even numbers. How can I do this?

I'm aware that there simpler, clever ways to do this. This is a contrived HR challenge, where the

The change_generator function that I wrote does not yield the desired output. I can only change change_generator .

I cannot change positive_integers_generator() nor the for loop below.

Can I solve this with a decorator?

#can't change the body of this function
def positive_integers_generator():
    n = 1
    while True:
        x = yield n
        if x is not None:
            n = x
        else:
            n += 1

# can only change this function            
def change_generator(generator, n):
  for i in generator:
    if i%2 == 0:
      yield(i)



# can't change this code either
# should print 1, 2, 4, 6, 8
g = positive_integers_generator() 
for _ in range(5):
    n = next(g)
    print(n)
    change_generator(g, n)

Answer 1

You can use the built in function filter

even_numbers_generator = filter(lambda n: n % 2 == 0, positive_integers_generator())

Or a generator expression.

even_numbers_generator = (n for n in positive_integers_generator() if n % 2 == 0)

Or itertools.count from the standard library:

even_numbers_generator = itertools.count(start=2, step=2)

But if you only can change the change_generator function, the "correct answer" to the challenge probably involves using generator.send()

# can only change this function            
def change_generator(generator, n):
    if n % 2 == 0:
        generator.send(n + 1)

Answer 2

You don't need the parens on generator in your loop, and you don't seem to be printing the output of the right generator. Updated version that works for me:

def positive_integers_generator():
    n = 1
    while True:
        x = yield n
        if x is not None:
            n = x
        else:
            n += 1

def change_generator(generator):
  for i in generator:
    if i%2 == 0:
      yield i

g = positive_integers_generator()

# should print 1, 2, 4
for _ in range(5):
  n = next(change_generator(g))
  print(n)

Answer 3

In your very specific problem, if you can't change the print(n) part then you are pretty cornered because you can't change the instance of generator g that was created for positive_integers_generator() .

In what may be a frowned upon answer, in this particular case you might want to update the global g to be reassigned to a new generator after that:

def change_generator(generator, n):
    def even_gen():
        n = 2
        while True:
            if n % 2 == 0:
                yield n
            else:
                yield
            n += 1
    global g                          # directly change the g referenced in main code
    if not g.__name__ == 'even_gen':  # change g if it is not even_gen
        g = even_gen()

# output:
# 1
# 2
# None
# 4
# None