在 python 中获取设置器状态

Question

I am looking to return a status when setting an instance variable. This return value will either indicate that the value was set or the value was invalid. For example:

class C():
    def __init__(self):
        self._attr = 1

    @property
    def attr(self):
        return self._attr

    @attr.setter
    def attr(self, val):
        if val < 10:
            self._attr = val
            return 'value set'
        else:
            return 'value must be < 10'

status = C().attr = 11 # should return failed
status = C().attr = 9 # should return worked

Adding return values to all my setters would be time-consuming and seems to be bad practice. Is there a better way to get a status from a setter?

The other solution I thought of was to write a "setSetter()" function that calls attr = val then checks if attr == val (which works like a status variable). However, it seems like there should be a better method.

Also, if there is a better way to structure the flow control so that attributes aren't set to invalid values I'm open to changing my strategy.

Thanks in advance for the help!

Answer 1

The standard way to indicate failure in Python and/or prevent code from reaching an invalid state is to raise an exception to indicate you've encountered a situation where you can't continue execution. This doesn't require you to modify the return type:

    @attr.setter
    def attr(self, val: int) -> None:
        if val < 10:
            self._attr = val
        else:
            raise ValueError('value must be < 10')

try:
    C().attr = 9  # works
    C().attr = 11 # raises ValueError and goes to nearest matching except
    C().attr = 13 # is never executed
except ValueError as e:
    print(e)      # prints 'value must be < 10'