[Python-ideas] A real life example of "given"

[Peter O'Connor]

On Thu, May 31, 2018 at 1:55 PM, Neil Girdhar <mistersheik at 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.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20180531/dbc49b8c/attachment.html>