5

I was messing around and noticed that the following code yields the value once, while I was expecting it to return a generator object. 

def f():
    yield (yield 1) 
f().next() # returns 1

def g():
    yield (yield (yield 1) 
g().next() # returns 1

My question is what is the value of the yield expression and also why are we allowed to nest yield expression if the yield expression collapses?

Durwasa Chakraborty
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    Related/Dup: [yield(x) vs. (yield(x)): parentheses around yield in python](//stackoverflow.com/q/37845338) – Aran-Fey Sep 09 '18 at 18:43

1 Answers1

7

The value of the yield expression after resuming depends on the method which resumed the execution. If __next__() is used (typically via either a for or the next() builtin) then the result is None. Otherwise, if send() is used, then the result will be the value passed in to that method.

So this:

def f():
    yield (yield 1)

Is equivalent to this:

def f():
    x = yield 1
    yield x

Which in this case (since you're not using generator.send()) is equivalent to this:

def f():
    yield 1
    yield None

Your code is only looking at the first item yielded by the generator. If you instead call list() to consume the whole sequence, you'll see what I describe:

def f():
    yield (yield 1)

def g():
    yield (yield (yield 1)) 


print(list(f()))
print(list(g()))

Output:

$ python3 yield.py 
[1, None]
[1, None, None]

If we iterate the generator manually (as you have), but .send() it values, then you can see that yield "returns" this value:

gen = f()
print(next(gen))
print(gen.send(42))

Output:

$ python3 yield_manual.py 
1
42
Jonathon Reinhart
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    It's not really true that `yield` returns `None`. Rather, it returns the value that you [`send`](https://stackoverflow.com/questions/19302530/python-generator-send-function-purpose) to the generator. (And simply iterating over a generator is like calling `send(None)`.) – Aran-Fey Sep 09 '18 at 18:45
  • Thanks @Aran-Fey, I've been trying to find the relevant part of the documentation to elaborate on that. – Jonathon Reinhart Sep 09 '18 at 18:47