[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part) (original) (raw)

Steven D'Aprano steve at pearwood.info
Sun Jul 1 02:22:29 EDT 2018

On Sat, Jun 30, 2018 at 11:32:03PM -0500, Tim Peters wrote:

[...]

So, no, gaining performance is not the motivation here. You already had a way to make it "run fast'. The motivation is the brevity assignment expressions allow while retaining all of the two-statement form's advantages in easier readability, easier reasoning, reduced redundancy, and performance.

I think the two of you (Tim and Nick) are in violent agreement :-)

In fairness to Nick performance is a motivation here, in the sense of "how can we avoid making redundant multiple calls to a function in an expression without splitting it into multiple statements?".

If performance were the only concern, then I agree with your point that we already have a solution. Simply refactor this:

process(expensive_call(arg), expensive_call(arg) + 1)

to this:

useful_value = expensive_call(arg)
process(useful_value, useful_value + 1)

But since performance is not the only concern, all the other factors you refer to (saving vertical space, DRY, side-effects, etc) count too.

Assignment expressions aren't valuable because they give us One Big Win. They're valuable because they give us Many Little Wins, which we (proponents of PEP 572) believe will outweigh the (minor) additional costs in complexity and opportunities for abuse.

The closest (in my opinion) assignment expressions comes to a One Big Win is to reduce the need for cascades of if... statements:

m = re.match(pattern, text)
if m:
    ...
else:
    m = re.match(other_pattern, text)
    if m:
        ...
    else:
        m = re.match(third_pattern, text)
        if m: ...

which wastes both vertical and horizontal space.

If that were the only win, I'd consider dedicated syntax for if/elif statements alone, but it isn't. Little wins include:

This last one is, I think, the only one where the postfix form reads better, but maybe I only say that because I'm more familiar with it:

area = sqrt(s*(s-a)*(s-b)*(s-c)) where s = (a+b+c)/2

-- Steve

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