On Wed, May 30, 2018 at 11:32 AM Peter O'Connor <peter.ed.ocon...@gmail.com>
wrote:
> In comparison, I think that := is much simpler.
>
>
> In this case that's true, but a small modification:
>
> updates = {
> y: do_something_to(potential_update)
> for x in need_initialization_nodes
> for y in [x, *x.synthetic_inputs()]
> if potential_update is not None
> given potential_update = command.create_potential_update(y)
> }
>
> Shows the flexibility of this given syntax vs ":="
>
> If we think of "given" as just inserting a line with variable-definitions
> before the preceding statement, it seems clear that:
>
> updates = {
> y: potential_update
> given potential_update = command.create_potential_update(y)
> for x in need_initialization_nodes
> for y in [x, *x.synthetic_inputs()]
> if potential_update is not None
> }
>
> Should raise a NameError: name 'potential_update' is not defined, and
>
> updates = {
> y: potential_update
> for x in need_initialization_nodes
> for y in [x, *x.synthetic_inputs()]
> given potential_update = command.create_potential_update(y)
> if potential_update is not None
> }
>
>
> Should raise a NameError: name 'y' is not defined.
>
The reason I want it like that for comprehensions is that I think of it as
equivalent to:
updates = {}
for x in need_initialization_nodes:
for y in [x, *x.synthetic_inputs()]:
potential_update = command.create_potential_update(y)
if potential_update is not None:
updates[y] = potential_update
But you're right that this would be a second addition to the grammar. One
addition 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
Best,
Neil
For safety it seems reasonable that if a variable is "given" in a
> comprehension, trying to refer to it (even if it defined in the enclosing
> scope) before the inner-definition will result in a NameError.
>
>
> On Wed, May 30, 2018 at 2:22 PM, Steven D'Aprano <st...@pearwood.info>
> wrote:
>
>> On Wed, May 30, 2018 at 02:42:21AM -0700, Neil Girdhar wrote:
>>
>> > With "given", I can write:
>> >
>> > potential_updates = {
>> > y: potential_update
>> > for x in need_initialization_nodes
>> > for y in [x, *x.synthetic_inputs()]
>> > given potential_update = command.create_potential_update(y)
>> > if potential_update is not None}
>>
>> I'm not sure if that would be legal for the "given" syntax. As I
>> understand it, the "given" syntax is:
>>
>> expression given name = another_expression
>>
>> but you've got half of the comprehension stuffed in the gap between the
>> leading expression and the "given" keyword:
>>
>> expression COMPREH- given name = another_expression -ENSION
>>
>> so I think that's going to be illegal.
>>
>>
>> I think it wants to be written this way:
>>
>> potential_updates = {
>> y: potential_update
>> for x in need_initialization_nodes
>> for y in [x, *x.synthetic_inputs()]
>> if potential_update is not None
>> given potential_update = command.create_potential_update(y)
>> }
>>
>>
>> Or maybe it should be this?
>>
>> potential_updates = {
>> y: potential_update
>> given potential_update = command.create_potential_update(y)
>> for x in need_initialization_nodes
>> for y in [x, *x.synthetic_inputs()]
>> if potential_update is not None
>> }
>>
>>
>> I'm damned if I know which way is correct. Either of them? Neither?
>>
>> In comparison, I think that := is much simpler. There's only one place
>> it can go:
>>
>> potential_updates = {
>> y: potential_update
>> for x in need_initialization_nodes
>> for y in [x, *x.synthetic_inputs()]
>> if (
>> potential_update := command.create_potential_update(y)
>> ) is not None
>> }
>>
>>
>> --
>> Steve
>> _______________________________________________
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