Possible to return two lists from a list comprehen

问题:

Is it possible to return two lists from a list comprehension? Well, this obviously doesn't work, but something like:

rr, tt = [i*10, i*12 for i in xrange(4)]

So rr and tt both are lists with the results from i*10 and i*12 respectively. Many thanks

回答1:

>>> rr,tt = zip(*[(i*10, i*12) for i in xrange(4)])
>>> rr
(0, 10, 20, 30)
>>> tt
(0, 12, 24, 36)

回答2:

According to my tests, creating two comprehensions list wins (at least for long lists). Be aware that, the best voted answer is slower even slower than traditional for loops. List comprehensions are faster and clearer.

python -m timeit -n 100 -s 'rr=[];tt = [];' 'for i in range(500000): rr.append(i*10);tt.append(i*12)' 
10 loops, best of 3: 123 msec per loop

> python -m timeit -n 100 rr,tt = zip(*[(i*10, i*12) for i in range(500000)])' 
10 loops, best of 3: 170 msec per loop

> python -m timeit -n 100 'rr = [i*10 for i in range(500000)]; tt = [i*10 for i in range(500000)]' 
10 loops, best of 3: 68.5 msec per loop

It would be nice to see comprehension lists supporting the creation of multiple lists at a time.

回答3:

It is possible for a list comprehension to return multiple lists if the elements are lists. So for example:

>>> x, y = [[] for x in range(2)]
>>> x
[]
>>> y
[]
>>>

The trick with zip function would do the job, but actually is much more simpler and readable if you just collect the results in lists with a loop.