On Wed, May 30, 2018 at 7:54 PM Steven D'Aprano <steve@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 _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
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