python - python 中的专用@property 装饰器

Question

我有几个类，每个类都有一些属性。所有属性的共同点是它们应该是数字属性。这似乎是使用 python 装饰器的理想场所，但我似乎无法确定正确的实现方式是什么。这是一个简单的例子:

class Junk(object):
    def __init__(self, var):
        self._var = var

    @property
    def var(self):
        """A numeric variable"""
        return self._var

    @var.setter
    def size(self, value):
        # need to make sure var is an integer
        if not isinstance(value, int):
            raise ValueError("var must be an integer, var = {}".format(value))
        self._var = value

    @var.deleter
    def size(self):
        raise RuntimeError("You can't delete var")

在我看来，应该可以编写一个装饰器来完成所有事情，以便将上面的内容转换为:

def numeric_property(*args, **kwargs):
    ...

class Junk(object):
    def __init__(self, var):
        self._var = var

    @numeric_property
    def var(self):
        """A numeric variable"""
        return self._var

这样新的 numeric_property 装饰器可以在许多类中使用。

Answer 1

@property 只是 Python 的 descriptor protocol 的一个特例，因此您当然可以构建自己的自定义版本。对于您的情况:

class NumericProperty:
    """A property that must be numeric.

    Args:
      attr (str): The name of the backing attribute.

    """

    def __init__(self, attr):
        self.attr = attr

    def __get__(self, obj, type=None):
        return getattr(obj, self.attr)

    def __set__(self, obj, value):
        if not isinstance(value, int):
            raise ValueError("{} must be an integer, var = {!r}".format(self.attr, value))
        setattr(obj, self.attr, value)

    def __delete__(self, obj):
        raise RuntimeError("You can't delete {}".format(self.attr))

class Junk:

    var = NumericProperty('_var')

    def __init__(self, var):
        self.var = var

正在使用中:

>>> j = Junk('hi')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/jonrsharpe/test.py", line 29, in __init__
    self.var = var
  File "/Users/jonrsharpe/test.py", line 17, in __set__
    raise ValueError("{} must be an integer, var = {!r}".format(self.attr, value))
ValueError: _var must be an integer, var = 'hi'
>>> j = Junk(1)
>>> del j.var
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/jonrsharpe/test.py", line 21, in __delete__
    raise RuntimeError("You can't delete {}".format(self.attr))
RuntimeError: You can't delete _var
>>> j.var = 'hello'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/jonrsharpe/test.py", line 17, in __set__
    raise ValueError("{} must be an integer, var = {!r}".format(self.attr, value))
ValueError: _var must be an integer, var = 'hello'
>>> j.var = 2
>>> j.var
2