On Wed, May 30, 2018 at 11:32 AM Peter O'Connor <peter.ed.oconnor@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 <steve@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 }
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