[Python-ideas] Temporary variables in comprehensions

[Stephan Houben]

Note that you can already do:

 [y + g(y) for x in range(10) for y in [f(x)]]

i.e. for y  in [expr]
does exactly what the OP wants.
No new syntax needed.

If you hang out on python-list , you'll soon notice
that many newbies struggle already with the list comprehension
syntax.
It's a mini-language which is almost, but not entirely,
exactly unlike normal Python code.
Let's not complicate it further.

Stephan


2018-02-15 10:53 GMT+01:00 Evpok Padding <evpok.padding at gmail.com>:

> For simple cases such as `[y + g(y) for y in [f(x) for x in range(10)]]`,
> I don't really see what the issue is, if you really want to make it
> shorter,
> you can ``[y + g(y) for y in map(f,range(10))]` which is one of the rare
> case where I like `map` more than comprehensions.
>
> For more complex case, just define a intermediate generator along the lines
> ```
> f_samples = (f(x) for x in range(10))
> [y+g(y) for y in f_samples]
> ```
> Which does exactly the same thing but
>   - Is more readable and explicit
>   - Has no memory overhead thanks to lazy evaluation
>     (btw, you should consider generators for your nested comprenshions)
>
> While I am sometimes in the same state of mind, wishing for variables in
> comprehensions seems to me like a good indicator that your code needs
> refactoring.
>
> Best,
>
> E
>
> On 15 February 2018 at 10:32, Jamie Willis <jw14896.2014 at my.bristol.ac.uk>
> wrote:
> >
> > I +1 this at surface level; Both Haskell list comprehensions and Scala
> for comprehensions have variable assignment in them, even between iterating
> and this is often very useful. Perhaps syntax can be generalised as:
> >
> > [expr_using_x_and_y
> >  for i in is
> >   x = expr_using_i
> >  for j in is
> >   y = expr_using_j_and_x]
> >
> > This demonstrates the scope of each assignment; available in main result
> and then every clause that follows it.
> >
> > Sorry to op who will receive twice, forgot reply to all
> >
> > On 15 Feb 2018 7:03 am, "fhsxfhsx" <fhsxfhsx at 126.com> wrote:
> >>
> >> As far as I can see, a comprehension like
> >> alist = [f(x) for x in range(10)]
> >> is better than a for-loop
> >> for x in range(10):
> >>   alist.append(f(x))
> >> because the previous one shows every element of the list explicitly so
> that we don't need to handle `append` mentally.
> >>
> >> But when it comes to something like
> >> [f(x) + g(f(x)) for x in range(10)]
> >> you find you have to sacrifice some readableness if you don't want two
> f(x) which might slow down your code.
> >>
> >> Someone may argue that one can write
> >> [y + g(y) for y in [f(x) for x in range(10)]]
> >> but it's not as clear as to show what `y` is in a subsequent clause,
> not to say there'll be another temporary list built in the process.
> >> We can even replace every comprehension with map and filter, but that
> would face the same problems.
> >>
> >> In a word, what I'm arguing is that we need a way to assign temporary
> variables in a comprehension.
> >> In my opinion, code like
> >> [y + g(y) for x in range(10) **some syntax for `y=f(x)` here**]
> >> is more natural than any solution we now have.
> >> And that's why I pro the new syntax, it's clear, explicit and readable,
> and is nothing beyond the functionality of the present comprehensions so
> it's not complicated.
> >>
> >> And I hope the discussion could focus more on whether we should allow
> assigning temporary variables in comprehensions rather than how to solve
> the specific example I mentioned above.
> >>
> >>
> >>
> >>
> >>
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