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One way to limit the type in an instance by referring to another field within the same instance is to use a validator function. This function will be called whenever a new value is assigned to the field that needs to be limited.

The validator function can access other fields within the same instance using the "this" keyword. It can then check the value of the other field and validate that the new value is within the allowed range or matches the required type.

For example, consider a class that has two fields "age" and "isSenior". The "isSenior" field should only be set to true if the "age" field is greater than or equal to 65. We can use a validator function to enforce this rule:

class Person:
    def __init__(self, age):
        self.age = age
        self.isSenior = False

    def validate_is_senior(self, value):
        if value and self.age < 65:
            raise ValueError("Cannot set isSenior to True for person under 65")
        return value

    @property
    def isSenior(self):
        return self._isSenior

    @isSenior.setter
    def isSenior(self, value):
        self._isSenior = self.validate_is_senior(value)

In this example, we define a "validateissenior" function that checks whether the "isSenior" field can be set to the given value. This function is called whenever the "isSenior" field is set, and it can access the "age" field using "self.age".

The "isSenior" field is defined as a property, which means that we can use the "@property" and "@isSenior.setter" decorators to define getter and setter functions for the field. In the setter function, we call the "validateissenior" function to validate the new value before setting the field.