14.6. Generator Send

Important

.send() method allows to pass value to the generator
data = yield will receive this "sent" value

14.6.1. Example

After running you have to send None value to begin processing

✘>>> def run():
...     print('Starting worker...')
...     while True:
...         work = yield
...         print(f'Processing {work}')
>>>
>>>
>>> worker = run()
>>> worker.send(None)
Starting worker...
>>>
>>> worker.send('job1')
Processing job1
>>>
>>> worker.send('job2')
Processing job2
>>>
>>> worker.send('job3')
Processing job3

✘>>> from inspect import isgenerator, isgeneratorfunction
>>>
>>>
>>> def run():
...     print('Starting worker...')
...     while True:
...         work = yield
...         print(f'Processing {work}')
>>>
>>> worker = run()
>>> worker.send(None)
Starting worker...
>>>
>>> worker.send(1)
Processing 1
>>>
>>> worker.send('hello')
Processing hello
>>>
>>> worker.send([1,2,3])
Processing [1, 2, 3]
>>>
>>> isgeneratorfunction(run)
True
>>> isgenerator(worker)
True

14.6.2. Why Send None?!

After running you have to send None value to begin processing
Sending anything other will raise TypeError

✘>>> def run():
...     print('Starting worker...')
...     while True:
...         work = yield
...         print(f'Processing {work}')
>>>
>>>
>>> worker = run()
>>>
>>> type(worker)
<class 'generator'>
>>>
>>> worker.send('hello')
Traceback (most recent call last):
TypeError: can't send non-None value to a just-started generator

14.6.3. Send Upstream Cascade

✘>>> def worker():
...     print('Starting worker...')
...     while True:
...         work = yield
...         print(f'Processing {work}')
>>>
>>> def run(gen):
...     gen.send(None)
...     while True:
...         x = yield
...         gen.send(x)
>>>
>>>
>>> result = run(worker())
>>> result.send(None)
Starting worker...
>>>
>>> for x in range(0,3):
...     result.send(x)
Processing 0
Processing 1
Processing 2