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

[spir]

On 02/13/2014 10:59 PM, Ram Rachum wrote:
> 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...

Such a pattern is rather common, eg in some kind of hand-baked parsing. (Say you 
have match funcs returning False if no match, True if match but you don't 
want/need the result, some "form" having a truth value of True if you need the 
form; then you are matching a pattern choice).

Solution 0: for a little and particular choice:

     x = expensive_computation_0()
     if not x:
         x = expensive_computation_1()
     if not x:
         x = expensive_computation_2()
     # assert(x)   # or whatever check
	
Alternative:
     x = expensive_computation_0() \
     or expensive_computation_1() \
     or expensive_computation_2()
     # assert(x)

Works due to lazy evaluation of logical operations.


Solution 1: for a big choice, or the general case:

     for comp in computations:
         x = comp()
         if x: break
     # assert(x)

However, one must notice that in some way we are here abusing the logical idea 
of truth value (of a result, specifically) to make it indcate the success of a 
computation. In other words, we are carrying two pieces of information 
(result+success) in the same single piece of data.
And the loop version only works if all comp's take no param, or the same one(s), 
or you have a parallel array of param sets (in parsing, all match funcs take 
source & index).

d