FW: [Python-Dev] conditional expressions?

[Huaiyu Zhu]

On Tue, 16 Oct 2001 13:29:09 -0400, Steve Holden <sholden at holdenweb.com> wrote:
>I particularly shudder for the newbie who must read something like the
>following (untested code warning: I'm not building an interpreter that can
>handle this stuff, because I'm hoping it will disappear <0.75 wink>):
>
>l = if a then [i for i in l1 if i % 2] else [i for i in l2 if not i % 2]
>
>Of course, this might also be cast as:
>
>l = [i for i in (if a then l1 else l2) if (if a then i % 2 else not i % 2)]
>
>This may not be the best way to implement such an expression, but if it can
>be done then someone will do it. It contrasts quite badly with:
>
>if a:
>    l = [i for i in l1 if i % 2]
>else:
>    l = [i for i in l2 if not i % 2]
>

This proposed if-then-else expression is not in the same league of list
comprehension.  The last example is a clear improvement to:

if a:
    l = []
    for i in l1:
        if i % 2: l.append(i)
else:
    l = []
    for i l2:
        if not i % 2: l.append(i)


In contrast, the example of if-then-else alone would be

l = []
for i in (if a then l1 else l2):
	if (if a then i % 2 else not i % 2):
		l.append(i)

which is not at all clear by itself, and not much an improvement to

l = []
if a: ll = l1
else: ll = l2
for i in ll:
	if a: c = i % 2
	else: c = not i % 2
	if c: l.append(i)

In any case, bundling compoments of a list is usually a better strategy than
bundling branches of conditions.


Huaiyu