List comprehensions and evaluation of elements therein

James Harris james.harris.1 at gmail.com
Wed Sep 23 20:12:37 CEST 2015


A list comprehension has various components. Anyone know when each of 
the elements is evaluated? In the form

  [v0 for v0 in expr0 if expr1]

If v0 appears in expr0 or expr1 the evaluation order matters.

I think of the above as being a rewrite of

  results = []
  for v0 in expr0:
    if expr1:
      results.append(v0)
  return results

Further,

  [v0, v2 for v0 in expr0 if expr1 for v2 in expr2 if expr3]

leads to

  results = []
  for v0 in expr0:
     if expr1:
        for v2 in expr2:
           if expr3:
              results.append((v0, v2))
  return results

First of all, is that a correct analog of the list comprehension?

With this latter expansion the values of v0 and v2 could appear in expr4 
or expr5. Again, the evaluation order would matter.

James



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