[Python-ideas] Temporary variables in comprehensions

[Evpok Padding]

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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