Temporary variables in list comprehensions

Paul Rubin no.email at nospam.invalid
Mon Jan 9 05:44:12 EST 2017


Steven D'Aprano <steve+comp.lang.python at pearwood.info> writes:
> [(expensive_calculation(x), expensive_calculation(x) + 1) for x in data]

   def memoize(f):
	cache = {}
	def m(x):
		if x in cache:
			return cache[x]
		a = f(x)
		cache[x] = a
		return a
	return m

   ec = memoize(expensive_calculation)
   ...  [(ec(x), ec(x) + 1) for x in data]

Or can write:

    @memoize
    def expensive_calculation(x): ....

Note the Haskell version of your listcomp would be:

  [(e, e+1) | x <- data_, let e = expensive_calculation x]

Maybe Python could get some version of that.  I've wanted it more than
once.  (I used "data_" because data is a Haskell keyword).


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