>
> 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.
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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