On Wed, Apr 11, 2018 at 10:50 AM, Paul Moore <p.f.moore@gmail.com> wrote: 
In particular, I'm happiest with the named moving_average() function,
which may reflect to some extent my lack of familiarity with the
subject area. I don't *care* how it's implemented internally - an
explicit loop is fine with me, but if a domain expert wants to be
clever and use something more complex, I don't need to know. An often
missed disadvantage of one-liners is that they get put inline, meaning
that people looking for a higher level overview of what the code does
get confronted with all the gory details.

I'm all in favour of hiding things away into functions - I just think those functions should be as basic as possible, without implicit assumptions about how they will be used.  Let me give an example:

----

Lets look at your preferred method (A): 

    def moving_average(signal_iterable, decay, initial=0):
        last_average = initial
        for x in signal_iterable:
            last_average (1-decay)*last_average + decay*x
            yield last_average

    moving_average_gen = moving_average(signal, decay=decay, initial=initial)

And compare it with (B), which would require the proposed syntax:

    def moving_average_step(last_average, x, decay): 
        return (1-decay)*last_average + decay*x

    moving_average_gen = (average:= moving_average_step(average, x, decay=decay) for x in signal from x=initial)

-----

Now, suppose we want to change things so that the "decay" changes with every step.  

The moving_average function (A) now has to be changed, because what we once thought would be a fixed parameter is now a variable that changes between calls.  Our options are:
- Make "decay" another iterable (in which case other functions calling "moving_average" need to be changed).
- Leave an option for "decay" to be a float which gets transformed to an iterable with "decay_iter = (decay for _ in itertools.count(0)) if isinstance(decay, (int, float)) else decay".  (awkward because 95% of usages don't need this.  If you do this for more parameters you suddenly have this weird implementation with iterators everywhere even though in most cases they're not needed).
- Factor out the "pure"  "moving_average_step" from "moving_average", and create a new "moving_average_with_dynamic_decay" wrapper (but now we have to maintain two wrappers - with the duplicated arguments - which starts to require a lot of maintenance when you're passing down several parameters (or you can use the dreaded **kwargs).

With approach (B) on the other hand, "moving_average_step" and all the functions calling it, can stay the same: we just change the way we call it in this instance to:

    moving_average_gen = (average:= moving_average_step(average, x, decay=decay) for x, decay in zip(signal, decay_schedule) from x=initial)

----

Now lets imagine this were a more complex function with 10 parameters.  I see these kind of examples a lot in machine-learning and robotics programs, where you'll have parameters like "learning rate", "regularization", "minibatch_size", "maximum_speed", "height_of_camera" which might initially be considered initialization parameters, but then later it turns out they need to be changed dynamically.  

This is why I think the "(y:=f(y, x) for x in xs from y=initial)" syntax can lead to cleaner, more maintainable code.



On Wed, Apr 11, 2018 at 10:50 AM, Paul Moore <p.f.moore@gmail.com> wrote:
On 11 April 2018 at 15:37, Peter O'Connor <peter.ed.oconnor@gmail.com> wrote:

> If people are happy with these solutions and still see no need for the
> initialization syntax, we can stop this, but as I see it there is a "hole"
> in the language that needs to be filled.

Personally, I'm happy with those solutions and see no need for the
initialisation syntax.

In particular, I'm happiest with the named moving_average() function,
which may reflect to some extent my lack of familiarity with the
subject area. I don't *care* how it's implemented internally - an
explicit loop is fine with me, but if a domain expert wants to be
clever and use something more complex, I don't need to know. An often
missed disadvantage of one-liners is that they get put inline, meaning
that people looking for a higher level overview of what the code does
get confronted with all the gory details.

Paul