Question

I am trying to understand some of the subtle details of python generators. One of the test programs I wrote to see if I could both send and read alternatively values to/from the same generator is the following:

def injector():
    while True:
        try:
            print 'a'
            v = yield
            print 'b', v
            yield v
            print 'c'
        except GeneratorExit:
            print 'exit'
            break

g = injector()

print 'send none'
g.send(None)
print 'send 2'
g.send(2)
print 'receiving'
v = g.next()
print 'received', v

g.close()

The expected output for this program is:

send none
a
send 2
b 2
receiving
received 2
c
a
exit

The output I get is:

send none
a
send 2
b 2
receiving
c
a
received None
exit

Now, obviously, the question is why am I getting the above output ? What is it that I did not understand about how generators work ?

Answer 1

Let me try to clarify:

def injector():
    while True:
        try:
            print 'a'
            v = yield
            print 'b', v
            yield v
            print 'c'
        except GeneratorExit:
            print 'exit'
            break

g = injector()

print 'send none'
g.send(None)

Here the coroutine is started. It executes until the first yield whose result is returned from .send(), but then discarded.

The coroutine prints a and yields nothing, thus None. So it is ok to discard.

print 'send 2'
g.send(2)

Here you send a 2 to the coroutine, making it continue where you left it. v = 2.

It prints 2 and yields v again. You would expect that from the g.send() call.
So after receiving and discarding v, you do

print 'receiving'
v = g.next()

Here you give control to the coroutine again, which prints c, then a, then yields None again, which you get here.

print 'received', v

thus prints None for v.

What you probably want is

g = injector()

print 'send none'
g.send(None)
print 'send 2'
v = g.send(2)
print 'received', v

g.close()

(Note that this last block can be written more cleanly and nicely as follows:

from contextlib import closing
with closing(injector()) as g:
    print 'send none'
    g.send(None)
    print 'send 2'
    v = g.send(2)
    print 'received', v

)