def average_above_zero (items):
  """
    Compute the average of an given array only for positive values.

    @type items: list
    @param items: List of positive values

    @rtype: float
    @return: Return the average of the list
  """

  # Don't compute if items isn't an List (array)
  if type(items) is not list:
    raise TypeError('Type of items param need to be \'list\' and not ' + str(type(items)))

  average = -1
  sum = 0
  items_length = len(items)

  # Don't compute if items is empty
  if items_length > 0:
    for item in items:
      if (type(item) is float or type(item) is int) and item >= 0:
        sum += float(item)
      else:
        # Remove 1 to items_length if item isn't an positive value
        items_length -= 1
    average = sum / items_length
  else:
    raise ValueError('Items list must not be empty')

  return average