How to perform input validation for read-only instance attributes?

A very similar question was posted here, but there are no accepted answers, no code examples and I don’t really like the idea of using an external library as suggested by the only one answer provided there.

The following code allows to define read-only instance attributes:

class Point:
    def __init__(self, x, y):
        self._x = x
        self._y = y

    @property
    def x(self):
        return self._x

    @property
    def y(self):
        return self._y

but I would like to validate the user inputs as well. I want to validate if x and y have the right type.

What is the most pythonic/elegant way for doing this? If I provide setters, the attributes are no more read-only.

Is performing the input validation within the constructor the only way to go?

Answer

Here’s an elegant and pythonic way to do it which uses a factory function to create the properties:

class ReadOnlyError(Exception):
    """Attempt made to assign a new value to something that can't be changed."""


# Based on recipe in book "Python Cookbook 3rd Edition" - section 9.21 -
# titled "Avoiding Repetitive Property Methods".
def readonly_typed_property(name, expected_type):
    storage_name = '_' + name

    @property
    def prop(self):
        return getattr(self, storage_name)

    @prop.setter
    def prop(self, value):
        if hasattr(self, storage_name):
            raise ReadOnlyError('{!r} is read-only!'.format(name))
        if not isinstance(value, expected_type):
            raise TypeError('{!r} must be a {!r}'.format(name, expected_type.__name__))
        setattr(self, storage_name, value)

    return prop


class Point:
    x = readonly_typed_property('x', int)
    y = readonly_typed_property('y', int)

    def __init__(self, x, y):
        self.x = x
        self.y = y


if __name__ == '__main__':
    try:
        p1 = Point(1, 2)
    except Exception as e:
        print('ERROR: No exception should have been raised for case 1.')
        print(e)
    else:
        print('As expected, NO exception raised for case 1.')

    print()
    try:
        p2 = Point('1', 2)
    except TypeError as e:
        print(e)
        print(f'As expected, {type(e).__name__} exception raised for case 2.')
    else:
        print('ERROR: expected TypeError exception not raised for case 2')

    print()
    try:
        p1.x = 42
    except Exception as e:
        print(e)
        print(f'As expected, {type(e).__name__} exception raised for case 3.')
    else:
        print('ERROR: expected ReadOnlyError exception not raised for case 3')