Temporary variables in list comprehensions

jmp jeanmichel at sequans.com
Tue Jan 10 05:35:31 EST 2017

On 01/09/2017 04:53 AM, Steven D'Aprano wrote:
> Suppose you have an expensive calculation that gets used two or more times in a
> loop. The obvious way to avoid calculating it twice in an ordinary loop is with
> a temporary variable:
> result = []
> for x in data:
>      tmp = expensive_calculation(x)
>      result.append((tmp, tmp+1))
> But what if you are using a list comprehension? Alas, list comps don't let you
> have temporary variables, so you have to write this:
> [(expensive_calculation(x), expensive_calculation(x) + 1) for x in data]
> Or do you? ... no, you don't!
> [(tmp, tmp + 1) for x in data for tmp in [expensive_calculation(x)]]
> I can't decide whether that's an awesome trick or a horrible hack...

In any situation, the double list comprehension (also used to flatten 
lists) is very difficult to read.

What about

for x in (f(d) for d in data):
    result.append(x, x+1)

There's a double for loop in the same line but the generator parenthesis 
help a lot. No lame tmp variable involved.


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