[Python-ideas] Where-statement (Proposal for function expressions)

[Ben Finney]

Gerald Britton <gerald.britton at gmail.com>
writes:

> Anyway, I like the "where" idea not because of real or imagined
> performance gains, but because of its cleanness when expressing
> problems.

(Speaking of cwclean expression, please don't top-post; instead, remove
irrelevant quoted material and respond beneath each remaining point
<URL:http://en.wikipedia.org/wiki/Posting_style#Inline_replying>.)

> A big use for me would be in list comprehensions. In one
> project I work on, I see things like:
> 
> for i in [item in self.someobject.get_generator() if self.important_test(item)]
> 
> and other really long object references.
> 
> which I would like to write:
> 
> mylist =  [item in f if g(item)] where:
>     f = self.someobject.get_generator()
>     g = self.important_test

I presume these two are supposed to be equivalent (and syntactically
valid), so I'll take it the first one should be something like::

    mylist = [item for item in self.someobject.get_generator() if self.important_test(item)]

> To my eyes, the first is harder to read than the second one.

That's largely attributable to the fact that you've got one alternative
all on a single line, and the other broken into more easily-readable
lines. I don't think the ‘where’ syntax is much help there.

I would write the single-statement version as::

    mylist = [
        item for item in self.someobject.get_generator()
        if self.important_test(item)]

which makes it far more readable. I argue that this does away with
pretty much any justification for your use case above.

-- 
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Ben Finney