[Python-Dev] Examples for PEP 572

[Devin Jeanpierre]

On Wed, Jul 4, 2018 at 11:04 AM Steven D'Aprano <steve at pearwood.info> wrote:
> Did you actually mean arbitrary simple statements?
>
> if import math; mylist.sort(); print("WTF am I reading?"); True:
>     pass

Yes. To quote PEP 572: "This is a tool, and it is up to the programmer
to use it where it makes sense, and not use it where superior
constructs can be used."

> A more reasonable suggestion would be to only allow *assignments*, not
> arbitrary simple statements. But that's another special case:

I don't agree that it is more reasonable, for exactly the reasons you
describe it to be surprising.

> with one or more semicolon-separated statements between the "if" and the
> condition:
>
>     if statement; statement; condition:
>
> If we stick to the rule that semicolons separate statements, that means
> we have:
>
>
>     if statement  # SyntaxError
>     statement     # okay
>     condition:    # SyntaxError
>
>
> If we don't want that, we need a new rule to treat semicolons
> differently inside if statements that they're treated elsewhere.

Yes. This is analogous to complaining that [1, 2, 3] should be a
syntax error because clearly this is a tuple with three elements:
"[1", "2", and "3]". In as far as it's a new parsing rule, it is a
"special case" indeed.

> If we applied this rule "allow statements separated by semicolons"
> everywhere, we'd get this:
[snip]

Nobody said anything about allowing semicolon-delimited statements in
arbitrary places in the grammar.

-- Devin