code: def find_pad_index(self, array):
          
          try:
              return list(array).index(self.pad_value)
          except ValueError:
              return len(array)
docs: Find padding index.
  
          Args:
              array (list): integer list.
  
          Returns:
              idx: padding index.
  
          Examples:
               >>> array = [1, 2, 0]
               >>> self.find_pad_index(array)
               2