What does a list comprehension do

[Marko Rauhamaa]

Antoon Pardon <antoon.pardon at rece.vub.ac.be>:

>     [ <expression> for <var> in <iter> ]
>
> would implicitly be rewritten as follows:
>
>     [ (lambda <var>: <expression>)(<var>) for <var> in <iter>]

Funny enough, that's how "list comprehensions" are created in Scheme:

   (map (lambda (i)
          (lambda (x) (* i x)))
        '(0 1 2 3))))

> There would no change on how lambdas work or functions or closures.

First of all, it's weird to spend any effort in trying to alter a very
special case. I don't recall having to generate a list of such
functions. In fact, I barely ever use lambda in Python; explicit def
statements are much more pleasing to the eye and are not restricted to
simple expressions.

Secondly, you'd lose the nice symmetry between for statements and
comprehensions/generators. For example:

   ( lambda x: i * x for i in range(4) )

corresponds to:

    for i in range(4):
        yield lambda x: i * x

Would you embed an extra lambda there, too?

How about:

    i = 0
    while i < 4:
        yield lambda x: i * x
        i += 1

or:

    for i in range(4):
        def f(x):
            return i * x
        yield f


Marko