[Python-Dev] Multiline with statement line continuation

[Steven D'Aprano]

On Tue, Aug 12, 2014 at 08:04:35AM -0500, Ian Cordasco wrote:

> I think by introducing parentheses we are going to risk seriously
> confusing users who may then try to write an assignment like
> 
> a = (open('spam') as spam, open('eggs') as eggs)

Seriously?

If they try it, they will get a syntax error. Now, admittedly Python's 
syntax error messages tend to be terse and cryptic, but it's still 
enough to show that you can't do that.

py> a = (open('spam') as spam, open('eggs') as eggs)
  File "<stdin>", line 1
    a = (open('spam') as spam, open('eggs') as eggs)
                       ^
SyntaxError: invalid syntax

I don't see this as a problem. There's no limit to the things that 
people *might* do if they don't understand Python semantics:

for module in sys, math, os, 
    import module

(and yes, I once tried this as a beginner) but they try it once, realise 
it doesn't work, and never do it again.

 
> Because it looks like a tuple but isn't and I think the extra
> complexity this would add to the language would not be worth the
> benefit. 

Do we have a problem with people thinking that, since tuples are 
normally interchangable with lists, they can write this?

from module import [fe, fi, fo, fum,
                    spam, eggs, cheese]


and then being "seriously confused" by the syntax error they receive? Or 
writing this?

from (module import fe, fi, fo, fum,
                    spam, eggs, cheese)


It's not sufficient that people might try it, see it fails, and move on. 
Your claim is that it will cause serious confusion. I just don't see 
that happening.


> If we simply look at Ruby for what happens when you have an
> overloaded syntax that means two different things, you can see why I'm
> against modifying this syntax. 

That ship has sailed in Python, oh, 20+ years ago. Parens are used for 
grouping, for tuples[1], for function calls, for parameter lists, class 
base-classes, generator expressions and line continuations. I cannot 
think of any examples where these multiple uses for parens has cause 
meaningful confusion, and I don't think this one will either.


[1] Technically not, since it's the comma, not the ( ), which makes a 
tuple, but a lot of people don't know that and treat it as if it the 
parens were compulsary.


-- 
Steven