[Python-ideas] Map-then-filter in comprehensions

[Paul Moore]

On 8 March 2016 at 22:21, Sjoerd Job Postmus <sjoerdjob at sjec.nl> wrote:
> As for yet another syntax suggestion (if we want to introduce
> something).
>
>     [y for x in numbers with abs(x) as y if y > 5]
>
> The benefit is that it reads quite natural: "with expr as name".
> Another benefit is that keywords get reused.

Agreed, this is a plausible suggestion. But rather than just providing
an example usage, it would be helpful to see the full syntax of
comprehension with the proposed addition. See
https://docs.python.org/3/reference/expressions.html#grammar-token-comprehension
- note that a comprehension can have arbitrarily many for and if
clauses, interspersed in any order as long as the first one is a
"for". I'm guessing you'd add "with <some_sort_of_expression> as
<target_list>" as simply a third option. But what would the semantics
be? I'm guessing that "with xxx as yyy" translates basically as a
statement "yyy = xxx" in the notional expansion described in that
section ("considering each of the for or if clauses a block...")

Assuming that is the proposed definition, a few questions arise:

1. Is the behaviour this would assign to "with <foo> as x with <bar>
as x" (i.e., repeated bindings of the same name) what we'd want? Is it
likely to cause confusion in practice?
2. Do we need to restrict the <target_list>? Consider that "with
something as global_var[0]" is allowed by the definition - that would
leak values out of the comprehension. This isn't new - the "for"
variable works like this already - but is it something that's more
likely to be abused than currently? Should we care? (Python doesn't
tend to prohibit people from writing bad code).

> However, that already has semantics outside a comprehension for
> context-managers.

This is indeed a concern, albeit not a huge one - the definition has
to point out that in the expansion "with" should be read as an
assignment (written backwards) not as a with statement. It's not an
impossible burden, but it is mildly inconsistent.

This is probably the syntax I prefer of the ones suggested so far. But
I still haven't seen any *really* convincing arguments that we need
anything new in the first place.

Paul