是否必须在def __init__中声明所有Python实例变量?

不必在__init__中声明它们，但是为了使用self设置实例变量，需要引用self，而您定义变量的位置则没有.

They do not have to be declared in __init__, but in order to set an instance variable using self, there needs to be a reference to self, and the place you are defining the variables does not.

但是，

class BinaryNode():
    parent = None
    left_child = None

    def run(self):
        self.parent = "Foo"
        print self.parent
        print self.left_child

输出将为

Foo
None

要在评论中回答您的问题，是的.在我的示例中，您可以说:

To answer your question in the comment, yes. You can, in my example say:

bn = BinaryNode()
bn.new_variable = "Bar"

或者，正如我所展示的，您可以设置一个类级别的变量.该类的所有新实例将在实例化时获得类级别变量的副本.

Or, as I showed, you can set a class level variable. All new instances of the class will get a copy of the class level variables at instantiation.

也许您不知道可以将参数传递给构造函数:

Perhaps you are not aware that you can pass arguments to the constructor:

class BinaryNode(object):

    def __init__(self, parent=None, left_child=None):
        self.parent = parent
        self.left_child = left_child



bn = BinaryNode(node_parent, node_to_the_left)