On Thu, May 31, 2018 at 1:55 PM, Neil Girdhar <mistersheik@gmail.com> wrote:

Why wouldn't you want to just put the outer given outside the entire comprehension?
retval = [expr(name, x) given name=update(name, x) for x in seq] given name=something

There seems to be a lot of controversy about updating variables defined outside a comprehension within a comprehension. Seems like it could lead to a lot of bugs and unintended consequences, and that it's safer to not allow side-effects of comprehensions.

The more I think about it, the more i want to keep "given" in comprehensions, and given in expressions using parentheses when given is supposed to bind to the expression first.

I think the problem is that you given to be used in two ways:

- You want "B given A" to mean "execute A before B" when B is a simple expression

- "given A B" to mean "execute A before B" when B is a loop declaration.

The more I think about it, the more I think comprehensions should be parsed in reverse order: "from right to left". In this alternate world, your initial example would have been:

potential_updates = {

(1) y: command.create_potential_update(y)

(2) if potential_update is not None

(3) given potential_update = command.create_potential_update(y)

(4) for y in [x, *x.synthetic_inputs()]

(5) for x in need_initialization_nodes

}

Which would translate to:

potential_updates = {}

(5) for x in need_initialization_nodes:

(4) for y in [x, *x.synthetic_inputs()]:

(3) potential_update = command.create_potential_update(y)

(2) if potential_update is not None:

(1) potential_updates[y] = command.create_potential_update(y)

And there would be no ambiguity about the use of given. Also, variables would tend to be used closer to their declarations.

But it's way to late to make a change like that to Python.