Simple example of flattening an array in python3

py3_flatten.py

def _flatten(values):
  """Recursively flattens a list using yield from"""
  for v in values:
    if (isinstance(v, list)):
      yield from _flatten(v) # https://www.python.org/dev/peps/pep-0380/
    else:
      yield v

def flattened_list(values):
  """Recursively flattens a list. If the passed in argument is not a list, an 
  error is thrown."""
  if isinstance(values, list):
    return list(_flatten(values))
  else:
    raise ValueError('Passed in value must be a list')

if __name__ == '__main__':
  print(flattened_list([[1,2,[3]],4]) == [1, 2, 3, 4])
  print(flattened_list([]) == [])
  print(flattened_list(None)) # error thrown, should be a list
  print(flattened_list("")) # error thrown, should be a list
  print(flattened_list("123123")) # error thrown, should be a list