code: def listify(a):
      
      if a is None:
          return []
      elif not isinstance(a, (tuple, list, np.ndarray)):
          return [a]
      return list(a)
docs: Convert a scalar ``a`` to a list and all iterables to list as well.
  
      Examples
      --------
      >>> listify(0)
      [0]
  
      >>> listify([1,2,3])
      [1, 2, 3]
  
      >>> listify('a')
      ['a']
  
      >>> listify(np.array([1,2,3]))
      [1, 2, 3]
  
      >>> listify('string')
      ['string']