List Partition Comprehension (Feature Suggestion)

[Yakov Shalunov]

Python list comprehension is substantially faster than plan iteration, to the point where
```
l0, l1 = [],[]
for x in l:
    if cond(x):
        l0.append(x)
    else:
        l1.append(x)
```
runs at about the same speed as
```
l0 = [x for x in l if cond(x)]
l1 = [x for x in l if not cond(x)]
```
assuming `cond` is a computationally light conditional.
While this isn't an extremely common use, I suggest a "partition comprehension" syntax which extends normal filtered-generator syntax. Such as:
```
l0, l1 = ([x for x in l if cond(x) else x])
```
(parenthesis there to indicate that the output is a tuple)
It could allow extension to n-way partitions as well. Because elif doesn't fit nicely in-line, it would probably be nicer to do it like:
```
l0, l1, l2 = ([x for x in l if cond0(x) else x**2 if cond1(x) else x**3])
```
So l0 would be all elements that match cond0, l1 would be squares of all that match cond1, and l2 would be cubes of everything that matches neither.