Python, working with list comprehensions

I have such code:

a = [[1, 1], [2, 1], [3, 0]]

I want to get two lists, the first contains elements of 'a', where a[][1] = 1, and the second - elements where a[][1] = 0. So

first_list = [[1, 1], [2, 1]] 

second_list = [[3, 0]]. 

I can do such thing with two list comprehension:

first_list = [i for i in a if i[1] == 1]

second_list = [i for i in a if i[1] == 0]

But mayb开发者_如何学Ce exists other (more pythonic, or shorter) way to do this? Thanks for your answers.

List comprehension are very pythonic and the recommended way of doing this. Your code is fine.

If you want to have it in a single line you could do something like

first_list, second_list = [i for i in a if i[1] == 1], [i for i in a if i[1] == 0]

Remember that, "Explicit is better than implicit."

Your code is fine

You can use sorted() and itertools.groupby() to do this, but I don't know that it would qualify as Pythonic per se:

>>> dict((k, list(v)) for (k, v) in itertools.groupby(sorted(a, key=operator.itemgetter(1)), operator.itemgetter(1)))
{0: [[3, 0]], 1: [[1, 1], [2, 1]]}

what about this,

In [1]: a = [[1, 1], [2, 1], [3, 0]]

In [2]: first_list = []

In [3]: second_list = []

In [4]: [first_list.append(i) if i[1] == 1 else second_list.append(i) for i in a]
Out[4]: [None, None, None]

In [5]: first_list, second_list
Out[5]: ([[1, 1], [2, 1]], [[3, 0]])

instead of two sublist, I prefer dict (or defaultdict, OrderedDict, Counter, etc.)

In [6]: from collections import defaultdict

In [7]: d = defaultdict(list)

In [8]: [d[i[1]].append(i) for i in a]
Out[8]: [None, None, None]

In [9]: d
Out[9]: {0: [[3, 0]], 1: [[1, 1], [2, 1]]}

If the lists are reasonably short then two list comprehensions will do fine: you shouldn't be worried about performance until your code is all working and you know it is too slow.

If your lists are long or the code runs often and you have demonstrated that it is a bottleneck then all you have to do is switch from list comprehensions to a for loop:

first_list, second_list = [], []
for element in a:
    if element[1] == 1:
        first_list.append(element)
    else:
        second_list.append(element)

which is both clear and easily extended to more cases.

list comprehensions are great. If you want slightly more simple code (but slightly longer) then just use a for loop.

Yet another option would be filters and maps:

a = [[1, 1], [2, 1], [3, 0]]
g1=filter(lambda i: i[1]==1,a)
g1=map(lambda i: i[0],g1)
g2=filter(lambda i: i[1]==0,a)
g2=map(lambda i: i[0],g2)
print g1
print g2