[Python-ideas] Introducing where clauses

[Andrey Popp]

Hello.

Reading discussion on python-ideas about  "Accessing the result of
comprehension's expression from the conditional", I've came to the
idea of where clauses, similar to Haskell's.

This solves the problem of recalculating of value multiple times. For
example, in the following expression:

    [(f(x), f(x)) for x in some_iterable if f(x) < 2]

value f(y) calculates three times -- It is a problem if function f
takes much time to compute its value or has side effects. If we would
have where clause, we can rewrite expression above with:

    [(y, y) for x in some_iterable if y < 2 where y = f(x)]

I think it is really useful. We can also expand this idea to lambdas
or maybe to introducing arbitrary scoping blocks of code.

Other thoughts:
  - Can we use where clauses in lambda definition to allow some kind
of multi-line lamdba's?