[Python-ideas] if expensive_computation() as x:

[Michele Lacchia]

You could gather all the functions in a list and then iterate and break
when needed:

for func in [f1, f2, f3, f4]:
    x = func()
    if x:
        # bla bla
        break

Sorry for the indentation if it's not right, I'm writing from my phone.
Il 14/feb/2014 03:40 "Ram Rachum" <ram.rachum at gmail.com> ha scritto:

> Hi everybody,
>
> Please excuse the recent torrent of crazy ideas :)
>
> I was reading code of the Shpaml project, trying to make a patch, while I
> saw code that looked inelegant to me. I considered how to improve it, but
> then saw I don't know how to.
>
> The code paraphrased is this:
>
>     if expensive_computation_0():
>         x = expensive_computation_0()
>         # Do something with x...
>     elif expensive_computation_1():
>         x = expensive_computation_1()
>         # Do something with x...
>     elif expensive_computation_2():
>         x = expensive_computation_2()
>         # Do something with x...
>
> The problem here is that we're doing the expensive computation twice.
> That's because we first need to check its truth value, and if it's true we
> need to actually use the value and do stuff with it.
>
> One solution would be to calculate it and put it in a variable before
> checking truth value. The problem with that is, besides being quite
> verbose, it doesn't work with the `elif` lines. You want to calculate the
> values only if the `elif` branch is being taken. (i.e. you can't do it
> before the `if` because then you might be calculating it needlessly since
> maybe the first condition would be true.)
>
> Obviously this can be "solved", i.e. rewritten in a way that wouldn't call
> the expensive operation twice, but the solution would probably be quite
> verbose, which is something we don't want.
>
> My suggestion:
>
>     if expensive_computation_0() as x:
>         # Do something with x...
>     elif expensive_computation_1() as x:
>         # Do something with x...
>     elif expensive_computation_2() as x:
>         # Do something with x...
>
> If you'd like to bind to a variable only a part of the condition, this
> would work too:
>
>     if x<5 with expensive_computation_0() as x:
>         # Do something with x
>
> What do you think?
>
>
> Ram.
>
>
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