Efficiently detect sign-changes in python

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What about:

import numpy
a = [1, 2, 1, 1, -3, -4, 7, 8, 9, 10, -2, 1, -3, 5, 6, 7, -10]
zero_crossings = numpy.where(numpy.diff(numpy.sign(a)))[0]

Output:

> zero_crossings
array([ 3,  5,  9, 10, 11, 12, 15])

I.e., zero_crossings will contain the indices of elements before which a zero crossing occurs. If you want the elements after, just add 1 to that array.

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