[Python-ideas] PEP 572: Statement-Local Name Bindings

[Matt Arcidy]

I appreciate that point as it is what I must be misunderstanding.

I believe the performance speed up of [((f(x) as h), g(h)) for x
range(10)] is that there are 10 calls to compute f, not 20.

You can do this with a dictionary right now, at least for the example
we're talking about:
[(d[x], g(d[x])) for x in range(10) if d.update({x:f(x)}) is None]

It's ugly but get's the job done.  The proposed syntax is leagues
better than that, but just to give my point a concrete example.

If f is memoized, isn't [( f(x), g(f(x)) ) for range(10)] the same?
You compute f 10 times, not 20.  You get the second f(x) as cache
retrieval instead of recomputing it, precisely because the argument x
is the same.

Here I can use 'd' later if needed, as opposed to with the proposal.
That's really my point about memoization (or cacheing).  If 'f' is
really expensive, I don't really see the point of using an ad-hoc
local caching of the value that lives just for one statement when I
could use it where-ever, even persist it if it makes sense.

I fully admit I'm at my depth here, so I can comfortably concede it's
better than memoization and I just don't understand!

On Wed, Feb 28, 2018 at 12:37 AM, Chris Angelico <rosuav at gmail.com> wrote:
> On Wed, Feb 28, 2018 at 6:46 PM, Matt Arcidy <marcidy at gmail.com> wrote:
>> I have been struggling to justify the need based on what I have read.  I
>> hope this isn't a dupe, I only saw caching mentioned in passing.
>>
>> Also please excuse some of the naive generalizations below for illustrative
>> purposes.
>>
>> Is there a reason memoization doesn't work? If f is truly expensive, using
>> this syntax only makes sense if you have few comprehensions (as opposed to
>> many iterations) and few other calls to f. Calling f in 10 comprehensions
>> would (naively) benefit from memoization more than this.  It appears to me
>> to be ad-hoc memoization with limited scope.  is this a fair statement?
>
> Memoization is only an option if the expression in question is (a) a
> single cacheable function call, and (b) used twice without any
> variation. If it's any sort of more complicated expression, that
> concept doesn't work.
>
> ChrisA
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