python中的专用@property装饰器

Question

我有几个类，每个类都有许多属性。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，但我似乎无法理解正确的实现是什么。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操作，以便将上述内容转换为：

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

选项1：继承自property

property 是一个描述符。见Descriptor HowTo on python.org。

因此，可以从property 继承并覆盖相关方法。

例如，在 setter 上强制执行 int：

class numeric_property(property):
    def __set__(self, obj, value):
        assert isinstance(value, int), "numeric_property requires an int"
        super(numeric_property, self).__set__(obj, value)

class A(object):
    @numeric_property
    def x(self):
        return self._x

    @x.setter
    def x(self, value):
        self._x = value

现在你已经强制执行整数了：

>>> a = A()
>>> a.x = 'aaa'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __set__
AssertionError: numeric_property requires an int

---

选项 2：创建更好的描述符

另一方面，实现一个不从属性继承的全新描述符可能会更好，这样您就可以一次性定义属性。

有这样的界面会更好：

class A(object):
    x = numeric_property('_x')

为此，您将实现一个带有属性名称的描述符：

class numeric_property(object):
    def __init__(self, private_attribute_name, default=0):
        self.private_attribute_name = private_attribute_name
        self.default = default

    def __get__(self, obj, typ):
        if not obj: return self
        return getattr(obj, self.private_attribute_name, self.default)

    def __set__(self, obj, value):
        assert isinstance(value, int), "numeric_property requires an int"
        setattr(obj, self.private_attribute_name, value)

---

免责声明 :)

我宁愿不在 Pyhon 中强制执行严格类型，因为没有它，Python 会更强大。

Answer 2

您可以只创建一个为您执行此操作的函数。尽可能简单，无需创建自定义描述符：

def numprop(name, privname):
    @property
    def _numprop(self):
        return getattr(self, privname)

    @_numprop.setter
    def _numprop(self, value):
        if not isinstance(value, int):
            raise ValueError("{name} must be an integer, {name} = {}".format(value, name=name))
        setattr(self, privname, value)

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


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

Answer 3

@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

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