[Python-ideas] If branch merging

[Andrew Barnert]

On Jun 7, 2015, at 20:41, Nick Coghlan <ncoghlan at gmail.com> wrote:
> 
>> On 8 June 2015 at 12:45, Chris Angelico <rosuav at gmail.com> wrote:
>>> On Mon, Jun 8, 2015 at 12:33 PM, Stephen J. Turnbull <stephen at xemacs.org> wrote:
>>> I'm definitely -1 on the also, alif syntax at this point.  On the
>>> other hand, having done a lot of C programming in my misspent youth, I
>>> do miss anaphoric conditionals, so I too would like to see the
>>> possibility of "if cond as var: do_something_with_var" explored.  Of
>>> course Nick is right that automatic common subexpression elimination
>>> (CSE) is the big win, but manual CSE can improve readability.
>> 
>> Part of the trouble with depending on CSE is that Python is so dynamic
>> that you can't depend on things having no side effects... but the more
>> important part, in my opinion, is that duplication is a source code
>> maintenance problem.
> 
> Yes, this is the part of the problem definition I agree with, which is
> why I think named subexpressions are the most attractive alternative
> presented in the past discussions.

The problem with general named subexpressions is that it inherently means a side effect buried in the middle of an expression. While it's not _impossible_ to do that in Python today (e.g., you can always call a mutating method in a comprehension's if clause or in the third argument to a function), but it's not common or idiomatic.

You could say this is a consulting-adults issue and you shouldn't use it in cases where it's not deep inside an expression--but those are the actual motivating cases, the ones where just "pull it out into a named assignment" won't work. In fact, one of our three examples is:

>    [b for a in iterable if (a.b as b)]

That's exactly the kind of place that you'd call non-idiomatic with a mutating method call, so why is a binding not even worse?

Maybe something more like a let expression, where the binding goes as far left as possible instead of as far right would look better, 
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