Python uses __slots__

Normally, when we define a class and create an instance of class, we can bind any property and method to that instance, which is the flexibility of dynamic language. Define class First:

>>> class Student(object):...     pass...

Then, try binding an attribute to the instance:

>>> s = Student()>>> s.name = ‘Michael‘ # 动态给实例绑定一个属性>>> print s.nameMichael

You can also try binding a method to an instance:

>>> def set_age(self, age): # 定义一个函数作为实例方法...     self.age = age...>>> from types import MethodType>>> s.set_age = MethodType(set_age, s, Student) # 给实例绑定一个方法>>> s.set_age(25) # 调用实例方法>>> s.age # 测试结果25

However, the method that is bound to one instance does not work for another instance:

>>> s2 = Student() # 创建新的实例>>> s2.set_age(25) # 尝试调用方法Traceback (most recent call last):  File "<stdin>", line 1, in <module>AttributeError: ‘Student‘ object has no attribute ‘set_age‘

To bind a method to all instances, you can bind the method to class:

>>> def set_score(self, score):...     self.score = score...>>> Student.set_score = MethodType(set_score, None, Student)

When you bind a method to a class, all instances are callable:

>>> s.set_score(100)>>> s.score100>>> s2.set_score(99)>>> s2.score99

Typically, the above set_score method can be defined directly in class, but dynamic binding allows us to dynamically add functionality to class while the program is running, which is difficult to implement in a static language.

Using __slots__

But what if we want to restrict the properties of class? For example, only add and attributes to student instances are allowed name age .

For the purpose of limiting, Python allows you to define a special variable when defining a class __slots__ to limit the attributes that the class can add:

>>> class Student(object):...     __slots__ = (‘name‘, ‘age‘) # 用tuple定义允许绑定的属性名称...

Then, let's try:

>>> s = Student() # 创建新的实例>>> s.name = ‘Michael‘ # 绑定属性‘name‘>>> s.age = 25 # 绑定属性‘age‘>>> s.score = 99 # 绑定属性‘score‘Traceback (most recent call last):  File "<stdin>", line 1, in <module>AttributeError: ‘Student‘ object has no attribute ‘score‘

Because the property cannot be bound because it is ‘score‘ not placed __slots__ in, the score attempt to bind score will get attributeerror error.

It __slots__ is important to note that the __slots__ defined properties work only on the current class and do not work on inherited subclasses:

>>> class GraduateStudent(Student):...     pass...>>> g = GraduateStudent()>>> g.score = 9999

Unless it is also defined in a subclass __slots__ , the subclass allows the defined property to be its own __slots__ plus the parent class __slots__ .

Python uses __slots__