Count consecutive occurences of values varying in length in a numpy array

If you already have a numpy array, this is probably going to be faster:

>>> condition = np.array([True,True,True,False,False,True,True,False,True])
>>> np.diff(np.where(np.concatenate(([condition[0]],
                                     condition[:-1] != condition[1:],
                                     [True])))[0])[::2]
array([3, 2, 1])

It detects where chunks begin, has some logic for the first and last chunk, and simply computes differences between chunk starts and discards lengths corresponding to False chunks.

Here's a solution using itertools (it's probably not the fastest solution):

import itertools
condition = [True,True,True,False,False,True,True,False,True]
[ sum( 1 for _ in group ) for key, group in itertools.groupby( condition ) if key ]

Out:
[3, 2, 1]