Python 3 用户定义的不可变类对象

Question

根据我的理解，Python 用户定义的类实例默认是不可变的。不可变对象(immutable对象)不会改变它们的哈希值，它们可以用作字典键和集合元素。

我有以下代码片段。

class Person(object):
    def __init__(self, name, age):
        self.name=name
        self.age=age

现在，我将实例化 Person 类并创建一个对象并打印其哈希值。

jane = Person('Jane', 29)
print(jane.__hash__())
-9223371933914849101

现在，我将改变 jane 对象并打印其哈希值。

jane.age = 33
print(jane.__hash__())
-9223371933914849101

我的问题是即使 jane 对象是可变的，为什么它的哈希值没有改变？

此外，我可以使用可变 jane 对象作为字典键和设置元素。

Answer 1

要定义具有不可变实例的类，您可以执行以下操作:

class Person:
    """Immutable person class"""

    # Using __slots__ reduces memory usage.
    # If __slots__ doesn't include __dict__, new attributes cannot be added.
    # This is not always desirable, e.g. it you want to subclass Person.
    __slots__ = ('name', 'age')

    def __init__(self, name, age):
        """Create a Person instance.

        Arguments:
            name (str): Name of the person.
            age: Age of the person.
        """
        # Parameter validation. This shows how to do this,
        # but you don't always want to be this inflexibe.
        if not isinstance(name, str):
            raise ValueError("'name' must be a string")
        # Use super to set around __setattr__ definition
        super(Person, self).__setattr__('name', name)
        super(Person, self).__setattr__('age', int(age))

    def __setattr__(self, name, value):
        """Prevent modification of attributes."""
        raise AttributeError('Persons cannot be modified')

    def __repr__(self):
        """Create a string representation of the Person.
        You should always have at least __repr__ or __str__
        for interactive use.
        """
        template = "<Person(name='{}', age={})>"
        return template.format(self.name, self.age)

一个测试:

In [2]: test = Person('S. Eggs', '42')

In [3]: str(test)
Out[3]: "<Person(name='S. Eggs', age=42)>"

In [4]: test.name
Out[4]: 'S. Eggs'

In [5]: test.age
Out[5]: 42

In [6]: test.name = 'foo'
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-6-1d0482a5f50c> in <module>()
----> 1 test.name = 'foo'

<ipython-input-1-efe979350b7b> in __setattr__(self, name, value)
     24     def __setattr__(self, name, value):
     25         """Prevent modification of attributes."""
---> 26         raise AttributeError('Persons cannot be modified')
     27 
     28     def __repr__(self):

AttributeError: Persons cannot be modified