How is the usage of classmethod in Python?

1 year ago
Jackson Davis
2 minutes

classmethod is a decorator used to declare a class method, which can be called either by the class itself or by an instance of the class.

In methods decorated with @classmethod, the first parameter is usually named “cls” to indicate the class itself. This parameter allows us to access class attributes and call other class methods within the class method.

Common uses of classmethod include:

  1. Create a factory method to instantiate a class without directly using its constructor.
  2. Provide an alternative way to construct a constructor that allows objects to be initialized in different ways.
  3. Using a class’s attributes and other class methods within a method without needing to instantiate an object.

Here is a simple example demonstrating the usage of classmethod:

class MyClass:
    count = 0
    
    def __init__(self, name):
        self.name = name
        MyClass.count += 1
    
    @classmethod
    def get_count(cls):
        return cls.count

# 调用类方法
print(MyClass.get_count())  # 输出: 0

# 创建类的实例
obj1 = MyClass("Object 1")
print(MyClass.get_count())  # 输出: 1

obj2 = MyClass("Object 2")
print(MyClass.get_count())  # 输出: 2

In the example above, we defined a class method called get_count that returns the value of the class attribute count. Calling this class method by the class name allows us to retrieve the count of objects created.

In class methods, it is possible to access the class properties using the cls parameter without instantiating the class object.

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