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

[Neil Girdhar]

On Wed, May 30, 2018 at 7:54 PM Steven D'Aprano <steve at pearwood.info> wrote:

> On Wed, May 30, 2018 at 01:59:37PM -0400, Neil Girdhar wrote:
>
> > This example shows additional flexibility:
> >
> > z = {a: transformed_b
> >      for b in bs
> >      given transformed_b = transform(b)
> >      for a in as_}
>
> Is that even legal?
>

In case you missed my earlier reply to you:

One addition to the grammar would be to "test" for something like

test: bool_test [comp_given]
bool_test: or_test ['if' or_test 'else' test] | lambdef
comp_given: 'given' testlist_star_expr annassign

The second would permit the usage in comprehensions:

comp_iter: comp_for | comp_if | comp_given


>
> Again, you're putting half of the comprehension in the middle of
> the given expression. I believe that "given" expression syntax is:
>
>     expression given name = another_expression
>
> it's not a syntactic form that we can split across arbitrary chunks of
> code:
>
>     # surely this won't be legal?
>     def method(self, arg, x=spam):
>         body
>     given spam = expression
>
>
> Comprehension syntax in this case is:
>
>     {key:expr for b in it1 for a in it2}
>
> (of course comprehensions can also include more loops and if clauses,
> but this example doesn't use those). So you've interleaved part of the
> given expression and part of the comprehension:
>
>     {key: expression COMPRE- given name = another_expression -HENSION}
>
>
> That's the second time you've done that. Neil, if my analysis is
> correct, I think you have done us a great service: showing that the
> "given" expression syntax really encourages people to generate syntax
> errors in their comprehensions.
>

> > There is no nice, equivalent := version as far as I can tell.
>
> Given (pun intended) the fact that you only use transformed_b in a
> single place, I don't think it is necessary to use := at all.
>
> z = {a: transform(b) for b in bs for a in as_}
>
> But if you really insist:
>
> # Pointless use of :=
> z = {a: (transformed_b := transform(b)) for b in bs for a in as_}
>
>
Those call transform for every a needlessly.


>
>
> --
> Steve
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