{"id":2626,"date":"2021-10-24T08:48:57","date_gmt":"2021-10-24T08:48:57","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=2626"},"modified":"2025-03-31T10:13:20","modified_gmt":"2025-03-31T10:13:20","slug":"python-__slots__","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-__slots__\/","title":{"rendered":"Python __slots__"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn about the Python __slots__<\/code> and how how to use it to make your class more efficient.<\/p>\n\n\n\n

Introduction to the Python __slots__ #<\/a><\/h2>\n\n\n\n

The following defines a Point2D<\/code> class that has two attributes including x<\/code> and y<\/code> coordinates:<\/p>\n\n\n

class<\/span> Point2D<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, x, y)<\/span>:<\/span>\n        self.x = x\n        self.y = y\n\n    def<\/span> __repr__<\/span>(self)<\/span>:<\/span>\n        return<\/span> f'Point2D({self.x}<\/span>,{self.y}<\/span>)'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
point = Point2D(0<\/span>, 0<\/span>)\nprint(point.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
To avoid the memory overhead, Python introduced the slots. If a class only contains fixed (or predetermined) instance attributes, you can use the slots to instruct Python to use a more compact data structure instead of dictionaries.<\/p>\n\n\n\n
For example, if the Point2D<\/code> class has only two instance attributes, you can specify the attributes in the slots like this:<\/p>\n\n\n
class<\/span> Point2D<\/span>:<\/span>\n    __slots__ = ('x'<\/span>, 'y'<\/span>)\n\n    def<\/span> __init__<\/span>(self, x, y)<\/span>:<\/span>\n        self.x = x\n        self.y = y\n\n    def<\/span> __repr__<\/span>(self)<\/span>:<\/span>\n        return<\/span> f'Point2D({self.x}<\/span>,{self.y}<\/span>)'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
By doing this, Python will not use the __dict__<\/code> for the instances of the class. The following will cause an AttributeError<\/code> error:<\/p>\n\n\n
point = Point2D(0<\/span>, 0<\/span>)\nprint(point.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
AttributeError: 'Point2D'<\/span> object has no attribute __dict__<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Instead, you’ll see the __slots__<\/code> in the instance of the class. For example:<\/p>\n\n\n
point = Point2D(0<\/span>, 0<\/span>)\nprint(point.__slots__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
('x'<\/span>, 'y'<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Also, you cannot add more attributes to the instance dynamically at runtime. The following will result in an error:<\/p>\n\n\n
point.z = 0<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
AttributeError: 'Point2D'<\/span> object has no attribute 'z'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Point2D.color = 'black'<\/span>\npprint(Point2D.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
mappingproxy({'__doc__'<\/span>: None<\/span>,\n              '__init__'<\/span>: <function Point2D.__init__ at 0x000001BBBA841310<\/span>>,\n              '__module__'<\/span>: '__main__'<\/span>,\n              '__repr__'<\/span>: <function Point2D.__repr__ at 0x000001BBBA8413A0<\/span>>,\n              '__slots__'<\/span>: ('x'<\/span>, 'y'<\/span>),\n              'color'<\/span>: 'black'<\/span>,\n              'x'<\/span>: <member 'x'<\/span> of 'Point2D'<\/span> objects>,\n              'y'<\/span>: <member 'y'<\/span> of 'Point2D'<\/span> objects>})<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
This code works because Python applies the slots to the instances of the class, not the class.<\/p>\n\n\n\n
Python __slots__ and single inheritance #<\/a><\/h2>\n\n\n\n
Let’s examine the slots in the context of inheritance.<\/p>\n\n\n\n
The base class uses the slots but the subclass doesn’t #<\/a><\/h3>\n\n\n\n
The following defines the Point2D<\/code> as the base class and Point3D<\/code> as a subclass that inherits from the Point2D<\/code> class:<\/p>\n\n\n
class<\/span> Point2D<\/span>:<\/span>\n    __slots__ = ('x'<\/span>, 'y'<\/span>)\n\n    def<\/span> __init__<\/span>(self, x, y)<\/span>:<\/span>\n        self.x = x\n        self.y = y\n\n    def<\/span> __repr__<\/span>(self)<\/span>:<\/span>\n        return<\/span> f'Point2D({self.x}<\/span>,{self.y}<\/span>)'<\/span>\n\n\nclass<\/span> Point3D<\/span>(Point2D)<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, x, y, z)<\/span>:<\/span>\n        super().__init__(x, y)\n        self.z = z\n\n\nif<\/span> __name__ == '__main__'<\/span>:\n    point = Point3D(10<\/span>, 20<\/span>, 30<\/span>)\n    print(point.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
{'z'<\/span>: 30<\/span>}<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The Point3D<\/code> class doesn’t have slots so its instance has the __dict__<\/code> attribute. In this case, the subclass Point3D<\/code> uses slots from its base class (if available) and uses an instance dictionary.<\/p>\n\n\n\n
If you want the Point3D<\/code> class to use slots, you can define additional attributes like this:<\/p>\n\n\n
class<\/span> Point3D<\/span>(Point2D)<\/span>:<\/span>\n    __slots__ = ('z'<\/span>,)\n\n    def<\/span> __init__<\/span>(self, x, y, z)<\/span>:<\/span>\n        super().__init__(x, y)\n        self.z = z<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Note that you don’t specify the attributes that are already specified in the __slots__<\/code> of the base class.<\/p>\n\n\n\n
Now, the Point3D<\/code> class will use slots for all attributes including x, y, and z.<\/p>\n\n\n\n
The base class doesn’t use __slots__ and the subclass doesn’t #<\/a><\/h3>\n\n\n\n
The following example defines a base class that doesn’t use the __slots__<\/code> and the subclass does:<\/p>\n\n\n
class<\/span> Shape<\/span>:<\/span>\n    pass<\/span>\n\n\nclass<\/span> Point2D<\/span>(Shape)<\/span>:<\/span>\n    __slots__ = ('x'<\/span>, 'y'<\/span>)\n\n    def<\/span> __init__<\/span>(self, x, y)<\/span>:<\/span>\n        self.x = x\n        self.y = y\n\n\nif<\/span> __name__ == '__main__'<\/span>:\n    # use both slots and dict to store instance attributes<\/span>\n    point = Point2D(10<\/span>, 10<\/span>)\n    print(point.__slots__)\n    print(point.__dict__)\n\n    # can add the attribute at runtime<\/span>\n    point.color = 'black'<\/span>\n    print(point.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
('x'<\/span>, 'y'<\/span>)\n{'color'<\/span>: 'black'<\/span>}<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this case, the instances of the Point2D<\/code> class uses both __slots__<\/code> and dictionary to store the instance attributes.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
However, you can add the class attributes<\/a> to the class:<\/p>\n\n\n
In this example, you assign an iterable<\/a> (a tuple<\/a>) that contains the attribute names that you’ll use in the class.<\/p>\n\n\n\n
By default, Python uses the dictionaries<\/a> to manage the instance attributes. The dictionary allows you to add more attributes to the instance dynamically at runtime. However, it also has a certain memory overhead. If the Point2D<\/code> class has many objects, there will be a lot of memory overhead.<\/p>\n\n\n\n
Each instance of the Point2D<\/code> class has its own __dict__<\/code> attribute that stores the instance attributes<\/a>. For example:<\/p>\n\n\n