[Python-ideas] Syntax: 'return: ...' expressions

[Andrew Barnert]

On Jan 5, 2015, at 10:55, Chris Angelico <rosuav at gmail.com> wrote:

> On Mon, Jan 5, 2015 at 8:49 PM, Andrew Barnert
> <abarnert at yahoo.com.dmarc.invalid> wrote:
>> Finally, are you sure your new return: isn't ambiguous without look ahead or context? When the parser reads "if spam: return", is it starting a return simple statement, or an expression statement that starts with a return expression? Have you tried actually writing out the grammar?
> 
> The way I see it, the return-expression is "return:", whereas the
> simple return statement would have no colon.

Are you suggesting that "return:" would be a single token (as opposed to all the other colons in the tokenizer), or that you'd use look ahead to disambiguate?

I suspect it probably _is_ possible to make something work that fits into Python's parser, but it isn't immediately obvious (at least to me) what that would be, which is why I wanted to see a grammar. (And also to know which "level" this comes at, to automatically answer questions like how it fits into things like conditional expressions and comprehension clauses and so forth.)