Lie Ryan wrote:
In list/generator comprehension, currently we have no way to access the result of the expression and have to write something like this:
[f(x) for x in l if f(x) > 0]
if f() is heavy or non-pure (i.e. have side effects), calling f() twice might be undesirable.
Even if f() is not a function such as
[x + 1 for x in l if x + 1 > 0]
it looks ugly since we're repeating ourself.
We can work around it like this:
[y for y in (f(x) for x in l) if y > 0]
but then we have an unnecessary nested loop comprehension.
I'm suggesting about something like:
[f(x) for x in l if @ > 0]
where @ is the result of the listcomp's expression (i.e. f(x))
Personally, I don't like the use of symbols like @, as python is not perl. I'm still thinking of a better syntax/approach and is open for suggestion.
What do you guys think?
IMO this is premature optimization. (I know that's an annoying thing to say, but bear with me...) Just using [f(x) for x in nums if f(x) > 0] is the most readable and obvious option. I think the case where such a line of code actually needs to be optimized is rare, and on there rare occasions using a slightly less readable variant is reasonable (along with an insightful comment). In other words, from my experience all we would gain from the proposed new syntax is making such premature optimization easier, at the cost of less readable code and more complex syntax. - Tal