Python Coroutines that receive and emits return values

Question

I am reading Python Essential Reference and I am unable to understand coroutine which receive and emits return values.

Here is what the author says - "A coroutine may simultaneously receive and emit return values using yield if values are supplied in the yield expression."

Here is an example that illustrates this:

def line_splitter(delimiter=None):
    print("Ready to split")
    result = None
    while True:
        line = (yield result)
        result = line.split(delimiter)

Further the author adds, In this case, we use the coroutine in the same way as before. However, now calls to send() also produce a result. For example:

>>> s = line_splitter(",")
>>> s.next()
Ready to split
>>> s.send("A,B,C")
['A', 'B', 'C' ]
>>> s.send("100,200,300")
['100', '200', '300']

I want to know how the above code works.

Thanks for any help.

Hard to answer unless you can tell us which part you don't understand. Do you have a good understanding of the generator aspect? — John La Rooy, Jul 22 '15 at 06:44
What I know about generators is that it uses yield keyword and produces a sequence of values used for iteration. I am not able to understand the working of above mentioned code. — user3379333, Jul 22 '15 at 06:50

score 4 · Accepted Answer · edited May 23 '17 at 12:34

Let's see what the calling code does, line by line:

s = line_splitter(",") This line just initializes the generator, without executing any of the code within it.
s.next() This executes the code up to and including the next yield statement, printing the line and yielding None. The assignment result = ..., however, is not executed yet.
s.send("A,B,C") This sets the "value" of yield within the generator to "A,B,C" and executes the code up to and including the next yield, thus assigning it to result.

In a sense, the yield keyword can be used for both, getting values out of the generator (using next) and at the same time injecting values into the generator (using send).

For a more in-depth explanation, you might also have a look at this answer.

Python Coroutines that receive and emits return values

1 Answers1