如何对只读实例属性执行输入验证？

Question

here发布了一个非常类似的问题，但是没有被接受的答案，也没有代码示例，我真的不喜欢使用外部库的想法，因为这里提供的答案只有一个。

以下代码允许定义只读实例属性：

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

但是我也想验证用户的输入。我想验证x和y是否有正确的类型。

做这件事的最有效的方法是什么？如果我提供setter，则属性不再是只读的。

在构造函数中执行输入验证是唯一的方法吗？

Answer 1

下面是一种优雅和节奏式的方法，它使用工厂函数来创建属性：

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')