[Python-ideas] On evaluating features [was: Unpacking iterables for augmented assignment]

[Stephen J. Turnbull]

Executive summary:  Much of this is my opinion, and as a newer poster,
your opinion is *more* valuable than mine (fresh eyes and all that).
What I hope you'll take from this post is a more precise understanding
of the customary criteria that are used in evaluating a proposed
feature on python-ideas and python-dev.

Apologies for breaking the thread, but per Subject it's not really part
of the thread.

James Lu writes:

 > I could, but I don't think that justifies not having this
 > functionality in python standard.

What's "this"?  Note:

 >> When replying, please edit your Subject line so it is more specific
 >> than "Re: Contents of Python-ideas digest..."

I had to edit the Subject.  I'm going to assume you mean

 >> 1. Re: Unpacking iterables for augmented assignment (Matthew Einhorn)

So.  The Python development community generally doesn't require
justification for refusing to implement new functionality.  Rather,
because Python has become a big and very complete programming
environment, and a fairly large language, implementing new syntax
requires that a feature increase expressiveness substantially.

In the case in point, the destructuring assignments

    a, b = b, a
    w, x, y, z = z, w, y, x

can be interpreted as "swapping" or "permuting", and AIUI that's why
they were included.  They express the intent better than

    tmp = a
    a = b
    b = tmp
    del tmp

and I don't want to even think about how to do the 4-variable version
without 4 temporary variables.  By comparison,

    x, y += a, b

is neither more expressive, nor easier to read, nor significantly
harder to type, than

    x += a
    y += b

as far as I can see.  Note that this is the "not harder to type"
criterion normally used in discussing new Python features, not
something I've invented.

 > This is something I think most students will expect while learning
 > python, especially if they're implementing algorithms.

I suppose this claim is unlikely to be accepted without testimony of
several teachers of Python.  In my own experience, I explicitly teach
my students that the destructuring assignment is *for* permuting, and
I have *not even once* been asked if it works for augmented
assignments.  By contrast, students with knowledge of other languages
(especially C-like languages) invariably "just use" augmented
assignments and ask if there's some spelling for increment and
decrement expressions.  Obviously, my teaching approach biases the
result, but if nobody ever overcomes that bias, I do not think it is
an expected or needed feature.

Steve