In the previous article in this series on transducers we looked at lazily evaluating transducers. This time we’ll look not at pulling output through a transducer chain from downstream, but at pushing input items into the chain from upstream.
All of the uses of transducers we’ve demonstrated in Python so far are probably better handled by existing and well established Python programming techniques, such as generator expressions and generator functions. At this point in the series, we move definitely beyond that into new territory where transducers bring completely new capabilities to Python.
One the key selling points of transducers is that they abstract the essence of a transformation away from the details of the data series that is being transformed. We’ll show this in Python by using transducers to transform a series of events modelled using Python coroutines.
Coroutines in Python
Coroutines in Python are little-used, and their workings are not widely known, so their implementation bears repeating here. If you’re familiar with the notion of coroutines in general, and the specifics of how they’re implemented in Python, you can skim over this section.
Coroutines are like generator functions insofar as they are resumable functions. In fact, coroutines in Python are generator functions which use
yield as an expression rather than a statement. What this means in practice is that generator function objects sport a
send() method which allows the client of the generator function to transmit information to a running generator and for the generator to receive this data as the value of the
yield expression. As usual, an example will serve to make things clearer.
We’ll start by defining a generator function which enters an infinite loop, waits at the
yield expression for a value to be received, and then prints this value to the console.
>>> def event_receiver(): ... while True: ... message = (yield) ... print("Message:", message) ... >>>
We create a generator object just the same as we would with any other generator:
>>> r = event_receiver() >>> r <generator object event_receiver at 0x100772828>
Now we’ll try to send it a message, using the
send() method of the generator object:
>>> r.send("message") Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: can't send non-None value to a just-started generator >>>
This actually fails, because the generator code has not yet been executed at all. We need to prime the pump, so to speak, by advancing execution to the first occurrence of
yield. We can do this by passing the generator to the
We’ll fix this pump-priming annoyance of generator based coroutines shortly.
Now we can send messages:
>>> r.send("message") Message: message >>> r.send("another message") Message: another message
When we’re done, we terminate the coroutine by calling the
close() method. (This actually raises a
GeneratorExit exception at the site of the
yield expression, which allows control flow to exit the otherwise infinite loop; this special exception is intercepted by the Python runtime system, so it isn’t seen by us at the console).
>>> r.close() >>>
Any further attempts to
send() messages into the generator function cause
StopIteration to be raised. This, of course, is the normal means of indicating that a generator is exhausted:
>>> r.send("message") Traceback (most recent call last): File "<stdin>", line 1, in <module> StopIteration >>>
Priming generator-based coroutines
Now to address the awkwardness of having to prime coroutine generator functions by initially passing them to
next(). We can improve this with a function decorator which creates the generator object and calls next on our behalf. We’ll call the decorator
def coroutine(func): def start(*args, **kwargs): g = func(*args, **kwargs) next(g) return g return start
We’ll use our new decorator to assist in defining a slightly more sophisticated coroutine for printing, called
import sys @coroutine def rprint(sep='\n', end=''): """A coroutine sink which prints received items to stdout Args: sep: Optional separator to be printed between received items. end: Optional terminator to be printed after the last item. """ try: first_item = (yield) sys.stdout.write(str(first_item)) sys.stdout.flush() while True: item = (yield) sys.stdout.write(sep) sys.stdout.write(str((item))) sys.stdout.flush() except GeneratorExit: sys.stdout.write(end) sys.stdout.flush()
In this implementation, we intercept
GeneratorExit explicitly to give us the opportunity to print a terminator. We also regularly flush the stream so we get immediate feedback for our following experiments.
The opposite of a sink is a source. Until now, we’ve been sourcing ‘events’ ourself by sending them from the REPL, but to make this a little more interesting, we’ll cook up a function – just a plain old function, not a generator – which takes values from an iterable series and intermittently sends them, after a delay, to anything with a
send() method such as our coroutine generators. For fun, the random delay has a so-called Poisson distribution which mimics a radioactive source; imagine a device with a geiger counter which sends the next item from an iterable series each time an atom decays:
def poisson_source(rate, iterable, target): """Send events at random times with uniform probability. Args: rate: The average number of events to send per second. iterable: A series of items which will be sent to the target one by one. target: The target coroutine or sink. Returns: The completed value, or None if iterable was exhausted and the target was closed. """ for item in iterable: duration = random.expovariate(rate) sleep(duration) try: target.send(item) except StopIteration as e: return e.value target.close() return None
When either the iterable series is exhausted or the target signals it has terminated (by raising
StopIteration) we call
close() on the target. Note that by supplying an infinite iterable series we could make the source send events forever.
Let’s hook our source and sink together at the REPL:
>>> printer = rprint(sep=', ', end='\nDONE!\n') >>> count_to_nine = range(10) >>> poisson_source(rate=0.5, iterable=count_to_nine, target=printer) 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 DONE! >>>
Combined event sources and event sinks
Of course, we can build functions which act as both sinks and sources, transforming the messages they receive in some way and forwarding the processed results onwards to another sink. Here’s a combined source and sink function, which simply doubles the values it receives:
@coroutine def doubler(target): while True: item = (yield) doubled_item = item * 2 try: target.send(doubled_item) except StopIteration as e: return e.value
doubler() we chain the components of the pipeline together
>>> printer = rprint(sep=', ', end='\nDONE!\n') >>> count_to_nine = range(10) >>> poisson_source(rate=0.5, ... iterable=count_to_nine, ... target=doubler(target=printer)) 0, 2, 4, 6, 8, 10, 12, 14, 16, 18 DONE!
doubler() it’s but a short hop to a more general
mapper() which accepts an arbitrary transforming function:
@coroutine def mapper(transform, target): while True: item = (yield) transformed_item = transform(item) try: target.send(transformed_item) except StopIteration as e: return e.value
Used like so,
>>> printer = rprint(sep=', ', end='\nDONE!\n') >>> count_to_nine = range(10) >>> poisson_source(rate=0.5, iterable=count_to_nine, target=mapper(transform=square, target=printer)) 0, 1, 4, 9, 16, 25, 36, 49, 64, 81 DONE!
From here, you can see how we could also implement equivalents of
reduce() and so on to operate on the ‘push’ event stream modelled by Python coroutines.
The point here is that we can’t just re-use any existing functions which process ‘pull’ data series – such as the functions in
itertools – with ‘push’ data series. Each and every function needs to be reimplemented to accept values pushed from upstream, and send processed results downstream.
Transducers provide a way out of this quandary. We’ve demonstrated earlier in this series that ‘reduce’ is a fundamental operation, and by reimagining
reduce() into a more general
transduce() we were able to use the same transducers to operate on both eager and lazy data series. We can do the same with coroutine-based push events, by implementing a version of
transduce() which allows us to use any transducer to process a stream of such events.
reactive_transduce() is a coroutine which accepts two arguments: a transducer and a target sink to which the transduced results will be sent:
@coroutine def reactive_transduce(transducer, target=None): reducer = transducer(sending()) accumulator = target if (target is not None) else reducer.initial() try: while True: item = (yield) accumulator = reducer.step(accumulator, item) if isinstance(accumulator, Reduced): accumulator = accumulator.value break except GeneratorExit: pass return reducer.complete(accumulator)
reactive_transduce() function connects to the upstream end of a transducer chain, adapting from the coroutine protocol to the reducer interface. At the downstream end of the transducer chain, we need to adapt the other way, from the reducer interface to the coroutine protocol. To do this we use a reducer called
Sending, which we hard-wire as the ‘bottom’ reducer on the first line of
Sending reducer looks like this:
class Sending: def initial(self): return null_sink() def step(self, result, item): try: result.send(item) except StopIteration: return Reduced(result) else: return result def complete(result): result.close() return result
step() method literally sends the next item to the
result – which must therefore be a legitimate event sink. Should the sink indicate that it can’t accept a further item, by raising
StopIteration we return the result wrapped in the
Reduced sentinel. The
initial() method provides a legitimate sink – just a simple do-nothing sink defined as:
@coroutine def null_sink(): while True: _ = (yield)
Going back to
reactive_transduce() the main loop continues to iterate, receiving new values via a yield expression, until such time as
GeneratorExit is signalled by the client or the reducer signals termination by returning
When the main loop is exited by whatever means, we give the reducer opportunity to
complete(), and the
Sending.complete() method ensures that
close() is called on the target.
With these pieces in place, let’s look at how to use
reactive_transduce(). We’ll reproduce our previous example where we squared the output from
poisson_source(), but this time using the
mapping() transducer to do the work:
>>> poisson_source(rate=0.5, ... iterable=range(10), ... target=transduce(transducer=mapping(square), ... target=printer)) ... 0, 1, 4, 9, 16, 25, 36, 49, 64, 81 DONE!
The key point here is that we can now take an arbitrary transducer and reuse it with eager collections, lazy iterables, and push-events! In fact, simply by devising an appropriate transduce function we can use re-use our transducers in an arbitrary data-series processing context.
This is the true power of transducers: Data processing components completely abstracted away from how the input data arrives, or to where the output results are sent.