provide additional constraints when declaring typing.NamedTuple object

Question

Using a typing.NamedTuple object, what is the best way to enforce additional constraints on how it can be declared?

Let's say I have a Undergraduate class where the students have have a major but I want to enforce that 'undeclared' is an unacceptable value for the major.

from typing import NamedTuple

class Undergraduate(NamedTuple):
    name: str
    major: str

    def check_major(self):
        if self.major == "undeclared":
            raise ValueError("must declare a major")

if __name__ == "__main__":
    u1 = Undergraduate("Jane", "computer science") # no errors
    u1.check_major() # no errors
    u2 = Undergraduate("John", "undeclared") # no errors
    u2.check_major() # ValueError

This works fine but I would like for check_major() to run every time I declare a new object, ie:

u1 = Undergraduate("John", "undeclared") # immediate ValueError raised

Is this possible using only a NamedTuple (I know how to do it using traditional classes)?

Note: I read this related question. These solutions provide somewhat of a working solution, but like the OP I want to be able to instantiate the objects without requiring additional class methods to be called.

Answer 1

NamedTuple protects both __init__ and __new__ from being replaced at declaration. However, they can be replaced after the class was created.

class Radial2D(NamedTuple):
      angle: float
      length: float

      def _verify_attributes_(self, *args):
          if self.length < 0 or not 0 < self.angle < 360:
             raise ValueError('Arguments out of range')

Radial2D.__init__ = Radial2D._verify_attributes_
print(Radial2D(90, 15.5))  # Radial2D(angle=90, length=15.5)
print(Radial2D(12, -5))    # ValueError: Arguments out of range

This pattern can be simplified using a class decorator:

from typing import Type, NamedTuple


def verify(tp: Type[NamedTuple]):
    verifier = tp._verify_attributes_
    tp.__init__ = verifier
    return tp


@verify
class Undergraduate(NamedTuple):
    name: str
    major: str

    def _verify_attributes_(self, *args):
        if self.major == "undeclared":
            raise ValueError("must declare a major")


print(Undergraduate("Jane", "computer science"))  # Undergraduate(name='Jane', major='computer science')
print(Undergraduate("John", "undeclared"))        # ValueError: must declare a major