[Python-Dev] Examples for PEP 572 (original) (raw)

Devin Jeanpierre jeanpierreda at gmail.com
Wed Jul 4 13:13:15 EDT 2018

On Wed, Jul 4, 2018 at 6:36 AM Steven D'Aprano <steve at pearwood.info> wrote:

On Wed, Jul 04, 2018 at 01:03:02AM -0700, Devin Jeanpierre wrote: > The PEP doesn't talk about it, but FWIW, Go and C++17 if statements > allow you to put arbitrary simple statements in the suite header, and > by doing that, solves all the issues you and the PEP mentioned. Whether they solve "all the issues" discussed in the PEP is doubtful, since they're entirely limited to just if statements.

The only issue that PEP 572 has with limiting to if statements is: "Disadvantages: Answers only a fraction of possible use-cases, even in if/whilestatements." The Go/C++17 syntax resolves that (while still being limited to if statements and while statements -- the primary use cases AFAIK, and the only ones Nathaniel agrees with).

> In Python it'd look like this: > > if x = expr(); x < 0:_ _> dostuff()

That's even more doubtful, since the semicolon in Python already has a meaning of separating statements on a line. That could be equivalent to: if x = expr() x < 0: dostuff() which would clearly have to be a syntax error. Two syntax errors. 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. Where LL(1) runs into trouble is that last statement must be an Expr statement, but that can be checked after parsing (e.g. during compilation). I don't understand why you think this is "clearly" a syntax error. (So is ":=", if you restrict yourself to the current Python syntax.)

As for ugly... well, compare:

while (kv := x.pop())[0] is not None: foo(kv[1])

while k, v = x.pop(); k is not None: foo(v)

if (v := d[k]) == _EOF: return None

if v = d[k]; v == _EOF: return None

I like ":=" when it is the only thing in the expression: "if a:= b:" etc. Every other time I would prefer C++/Go's syntax.

-- Devin

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