python子类中的RecursionError

Question

我编写了继承dict内置类型的python类。我想编写一种带有额外检查方法的容器。

如果我创建该类的一个实例，就会得到一个RecursionError。我不明白这个错误。

class Params(dict):
    """ A convenient container of all paramters """

    # list of authorized keywords with default values
    keywords = {"k1": 1, "k2": 2, "k3": 3}

    def __init__(self, *args, **kwargs):
        # self.update(self.keywords)
        for k, v in self.keywords.items():
            self[k] = v
        self.update(*args, **kwargs)

    def update(self, *args, **kwargs):
        for k, v in dict(*args, **kwargs).items():
            self[k] = v

    def __setitem__(self, name, value):
        """ set a parameters following dict syntax """
        name = name.lower()
        if name in self.keywords:
            self[name] = value
        else:
            raise KeyError("Keywords %s does not exist." % name)

    def __getattr__(self, name):
        """ get paramters as an attribute of the class """
        return self.__getitem__(name)

Answer 1

因此，正如我在注释中所暗示的，发生递归错误是因为您在self.__setitem__中隐含地调用__setitem__，因此应该调用超类__setitem__。有一种方法：

In [6]: class Params(dict):
   ...:     """ A convenient container of all paramters """
   ...:
   ...:     # list of authorized keywords with default values
   ...:     keywords = {"k1": 1, "k2": 2, "k3": 3}
   ...:
   ...:     def __init__(self, *args, **kwargs):
   ...:         # self.update(self.keywords)
   ...:         for k, v in self.keywords.items():
   ...:             self[k] = v
   ...:         self.update(*args, **kwargs)
   ...:
   ...:     def update(self, *args, **kwargs):
   ...:         for k, v in dict(*args, **kwargs).items():
   ...:             self[k] = v
   ...:
   ...:     def __setitem__(self, name, value):
   ...:         """ set a parameters following dict syntax """
   ...:         name = name.lower()
   ...:         if name in self.keywords:
   ...:             # note: dict.__setitem__(self, name, value) would also work
   ...:             super().__setitem__(name, value)
   ...:         else:
   ...:             raise KeyError("Keywords %s does not exist." % name)
   ...:
   ...:     def __getattr__(self, name):
   ...:         """ get paramters as an attribute of the class """
   ...:         return self.__getitem__(name)
   ...:

In [7]: p = Params(k1=10)

In [8]: p
Out[8]: {'k1': 10, 'k2': 2, 'k3': 3}

注意，我使用了Python3版本的super Python 2将需要：super(Params, self).__setitem__(name, value)

注意，一般来说，我更喜欢组合而不是继承。拥有一个具有内部dict属性的dict对象，您可以通过公共接口管理该对象。这将更加健壮，并避免从内置类型继承带来的麻烦。