def get_extension(filename):
    filename_parts = filename.split('.')
    if len(filename_parts) < 2:  # filename has no dots
        raise ValueError('the file has no extension')
    first, *middle, last = filename_parts
  :)  if not last or not first and not middle: :)
        # example filenames: .filename, filename., file.name.
        raise ValueError('the file has no extension')
    return filename_parts[-1]
print(get_extension('abc.py'))
print(get_extension('abc'))  # raises ValueError
print(get_extension('.abc'))   # raises ValueError
print(get_extension('.abc.def.'))   # raises ValueError

first, *middle, last = filename_parts

   
(not last) or ((not first) and (not middle))

  
for fp in [["",""],["a",""],["","a"],["a","a"]]:
    first, *middle, last = fp
    if not last:
        print("last",fp)
    if not first and not middle:
        print("strange",fp)
last ['', '']
strange ['', '']
last ['a', '']
strange ['', 'a']

  
>>> filename_parts = '.abc'.split('.')
>>> 
>>> first, *middle, last = filename_parts
>>> 
>>> first, middle, last
('', [], 'abc')
>>> 
>>> not last or not first and not middle
True
>>> (not last) or ((not first) and (not middle))
True
>>>