Following code should work as expected

In [1]: from typing import NamedTuple

In [2]: class X(NamedTuple):
   ...:     x: int
   ...:     def double(self):
   ...:         return 2 * x
   ...:     

In [3]: X(1).double()

At the moment it throws AttributeError as there is no double method on the constructed namedtuple.

The workaround is to define methods on subclass and set __slots__ = (). But unless I'm missing something, there is no reason why methods shouldn't work as expected.