python - Flatten a nested list of variable sized sublists into a SciPy array

Question

How can I use numpy/scipy to flatten a nested list with sublists of different sizes? Speed is very important and the lists are large.

 lst = [[1, 2, 3, 4],[2, 3],[1, 2, 3, 4, 5],[4, 1, 2]]

Is anything faster than this?

 vec = sp.array(list(*chain(lst)))

Answer 1

How about np.fromiter:

In [49]: %timeit np.hstack(lst*1000)
10 loops, best of 3: 25.2 ms per loop

In [50]: %timeit np.array(list(chain.from_iterable(lst*1000)))
1000 loops, best of 3: 1.81 ms per loop

In [52]: %timeit np.fromiter(chain.from_iterable(lst*1000), dtype='int')
1000 loops, best of 3: 1 ms per loop