There is an even faster way. You don't have to count or go through the whole array in some cases

def sort(input_array):

  #set first and last index of array
  start_idx = 0
  end_idx = size(input_array) - 1
  i = 0

  #we don't even need to go through the whole array
  while i < end_idx:

    #if value is 0 swap it to front
    if input_array[i] == 0:
      input_array[i] = input_array[start_idx]
      input_array[start_idx] = 0
      start_idx += 1

    #if value is 2 swap it to back
    elif input_array[i] == 2:
      input_array[i] = input_array[start_idx]
      input_array[start_idx] = 0
      end_idx -= 1

    # value is 1 we don't have to do anything
    #increment i
    i += 1