[Python-ideas] A real life example of "given"

[Steven D'Aprano]

On Thu, May 31, 2018 at 04:06:51AM +1000, Chris Angelico wrote:
> On Thu, May 31, 2018 at 3:59 AM, Neil Girdhar <mistersheik at gmail.com> wrote:
> > This example shows additional flexibility:
> >
> > z = {a: transformed_b
> >      for b in bs
> >      given transformed_b = transform(b)
> >      for a in as_}
> >
> > There is no nice, equivalent := version as far as I can tell.
> 
> True. However, it took me several readings to understand what you were
> doing here.

Possibly you shouldn't have tried reading at 4am.

Either that or I shouldn't be reading before I've had a coffee :-)

Have I missed something that you have seen? Even if the syntax were 
legal, that seems to be a pointless use of an assignment expression. 
Since the new name "transformed_b" is only used once, we can and should 
just use the transform(b) in place:

z = {a: transform(b) for b in bs for a in as_}

If we need to use it twice, we can do this:

# assume "@" stands in for something useful
z = {a: (transformed_b := transform(b)) @ transformed_b 
     for b in bs for a in as_}


I'm not seeing the advantage of given, or any extra flexibility here, 
unless the aim is to encourage people to make syntax errors :-)

What have I missed?



-- 
Steve