[Python-ideas] with-statement syntactic quirk

Nick Coghlan ncoghlan at gmail.com
Wed Oct 31 14:22:42 CET 2012

On Wed, Oct 31, 2012 at 10:52 PM, Eli Bendersky <eliben at gmail.com> wrote:
> On Wed, Oct 31, 2012 at 5:45 AM, Devin Jeanpierre <jeanpierreda at gmail.com>
> wrote:
>> Anyway, it looks like this isn't how the tokenizer treats
>> braces/brackets (it ignores indent/dedent, but not newlines (I guess
>> the grammar handles those)). What I meant to suggest was, treat "with
>> ... :" similarly to how the OP suggests treating "with (...) :".
> If this gets accepted, then, is there a reason to stop at "with"? Why not
> ignore newlines between "if" and its trailing ":" as well? [playing devil's
> advocate here]

Note that I agreed with Barry that we probably *should* change it from
a principle-of-least-surprise point of view. I just called "not it" on
actually figuring out how to make it work given the current Grammar
definition as a starting point :)

Between expression precedence control, singleton tuples, generator
expressions, function calls, function parameter declarations, base
class declarations, import statement grouping and probably a couple of
other cases that have slipped my mind, parentheses already have plenty
of different meanings in Python, and we also have plenty of places
where the syntactical rules aren't quite the same as those in an
ordinary expression.

The thing that makes Python's parser simple is the fact that we have
*prefixes* in the Grammar that make it clear when the parsing rules
should change, so you don't need much lookahead at parsing time (it's
deliberately limited to only 1 token, in fact). The challenge in this
particular case is to avoid a Grammar ambiguity relative to ordinary
expression syntax without duplicating large sections of the grammar


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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