Fwd: Re: Postfix conditionals

[Göktuğ Kayaalp]

This was sent to me as a private reply to a question that I have posted
to python-list at python.org, so I am forwarding it to here.

Chris, please send your messages to the list, and cc the OP.

-------- Original Message --------
Subject: 	Re: Postfix conditionals
Date: 	Sun, 5 Jan 2014 14:09:14 -0800
From: 	Chris Rebert <clp2 at rebertia.com>
To: 	Göktuğ Kayaalp <self at gkayaalp.com>
CC: 	Python <python-list at python.org>



On Sun, Jan 5, 2014 at 12:24 PM, Göktuğ Kayaalp <self at gkayaalp.com> wrote:
> Hi,
>
> AFAIK, we do not have "postfix conditionals" in Python, i.e. a condition
> appended to a
> statement, which determines whether the statement runs or not:
>
>   py> for i in [False]:
>   ...     break if not i
>
> The above piece of code is equivalent to this in Python:
>
>   py> for i in [False]:
>   ...    if not i
>   ...        break
>
> I believe that the first example is superior to the second example when the
> two is compared
> for readability and intuitiveness.

I'm going to have to disagree. I dislike how this obscures the
if-statement, complicates the language grammar, and adds another
unnecessary way to express the same thing (which violates TOOWTDI)
with little countervailing benefit.

> We already have a ternary statement that
> looks similar,
>
>   py> print('hi') if True else None

Actually, to be pedantic, it's a ternary *expression*. Using it purely
for side-effects (i.e. as a statement) is rather unidiomatic, in the
same way that abusing list comprehensions, e.g.:

     [print(i) for i in range(42)]

is frowned upon.
Not to mention that the ternary doesn't work for actual statements
(print() is just a function call in Python 3):

     >>> (x = 1) if True else (x = 2)
       File "<stdin>", line 1
         (x = 1) if True else (x = 2)
            ^
     SyntaxError: invalid syntax

> so I reckon there would be no breakage in old code if this kind of syntax
> was added.  Ruby has
> this, and AFAIK Perl also does.
>
> I lack the knowledge of whether the community has opinions on this kind of
> notation, so I am
> posting this here instead of the ideas list.  What are your thoughts on
> this?

You can already write:

     for i in [False]:
         if not i: break

if you feel the need for terseness or a one-liner. Perhaps this
satisfies your desire?

Cheers,
Chris



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