On Fri, Aug 11, 2017 at 02:34:53PM +1000, Nick Coghlan wrote:
> This is actually how I came to the conclusion that if we were ever to
> do something like this, the termination condition would need to go
> *before* the filter condition:
What if you want to check the filter condition before the termination
condition?
I have an iterable of arbitrary objects. I want to ignore anything that
isn't a string, and halt if the string doesn't start with "A". This is
easy:
[expr for s in objects if isinstance(s, str) while s.startswith("A")]
Why should we prohibit expressing this, and instead write it as this?
[expr for s in objects while (s.startswith("A")) if isinstance(s, str) else
True) if isinstance(s, str)]
Or split into multiple comprehensions?
[expr for s in [obj for obj in objects if isinstance(obj, str)] while
s.startswith("A")]
> (expr for var in seq while loop_cond if filter_cond)
>
> <=>
>
> for var in seq:
> if loop_cond:
> if filter_cond:
> yield expr
> else:
> break
We can still expand the clauses if they are presented in the opposite
order:
(expr for var in seq if filter_cond while loop_cond)
<=>
for var in seq:
if filter_cond:
if loop_cond:
yield expr
else:
break
There's no need to prohibit that. It is meaningful and useful and just
because somebody might accidentally fail to exit an infinite loop is no
reason to ban this.
> This is another good reason why a termination condition would need to
> be checked before the filter condition rather than either after it, or
> only when the filter condition was true.
Why is this a problem that needs solving?
Who is to say that an infinite generator expression isn't exactly what
the programmer wants? If the halting condition is not true, the
generator expression will either keep going until the iterator is
exhausted, or it will be an infinite generator just like the
unprocessed, unfiltered source iterator. This is not necessarily a
problem.
--
Steve
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