如何对只读实例属性执行输入验证？

Question

here 发布了一个非常相似的问题，但没有可接受的答案，没有代码示例，而且我不太喜欢那里提供的唯一答案所建议的使用外部库的想法。

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

但我也想验证用户输入。我想验证x 和y 的类型是否正确。

这样做最pythonic/优雅的方式是什么？如果我提供设置器，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

这是一种使用工厂函数创建属性的优雅和 Pythonic 方式：

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')