[Python-Dev] Examples for PEP 572

[Steven D'Aprano]

On Wed, Jul 04, 2018 at 10:13:15AM -0700, Devin Jeanpierre wrote:

> > > In Python it'd look like this:
> > >
> > > if x = expr(); x < 0:
> > >   do_stuff()
[...]

> > Python's parser is restricted to LL(1) by design, so treating semicolons
> > in "if" statements as a special case might not even be possible.
> > But even if it is possible, not only would this special case
> > break the Zen, but it would be UGLY too.
> 
> It isn't a special case, and it's still LL(1). Once you encounter an
> "if", you parse n simple statements separated by a ";", until you
> reach a colon.

I'll take your word about the LL(1).

Did you actually mean arbitrary simple statements?

if import math; mylist.sort(); print("WTF am I reading?"); True:
    pass


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

- everywhere else where a semicolon is legal, it can separate
  arbitrary simple statements;
- except inside an if clause, where only assignments are allowed.



> I don't understand why you think this is "clearly" a syntax error.

Taking the principle that semicolons separate statements, you have an 
if statement 

    if condition:

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.

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

    # I trust you don't want anything even close to these
    from x=1; sys import y=2; argv
    data x=1; y=2; = sorted(data)
    raise x=1; y=2; Exception as err:


So we have to treat semicolons inside if statements differently from 
semicolons everywhere else. That what I meant by saying it would be a 
special case.

The beauty of assignment expressions is that they AREN'T a special case. 
Being an expression, they're legal anywhere any other expression is 
allowed, and illegal anywhere where expressions aren't allowed.


-- 
Steve