A class method is a method that is bound to a class rather than an instance, defined with @classmethod, and the first parameter is the class (cls). It can be called through a class or instance, and is suitable for creating factory methods, modifying class status, and encapsulating class-level operations. For example, use class methods to implement instance creation in different data formats, or automatically identify the calling class in inheritance. Be careful to avoid accessing instance properties. It is recommended to call through class names to clarify the intent.
It is actually not difficult to call Python class methods, but it is easy for novices to confuse the usage scenarios of different methods. Simply put, a class method is a method that binds to a class rather than an instance, which is usually defined with @classmethod decorator, and the first parameter is the class itself (usually writing cls ).
What is a class method?
A class method is different from a normal method (instance method). It belongs to a part of a class, not an instance of a class. You can call it through a class or an instance, but either way, the first parameter is always the class itself.
For example:
class MyClass:
@classmethod
def show_class(cls):
print(f"Called from {cls.__name__}") Whether you use MyClass.show_class() or my_instance.show_class() , the output is the same.
How to define and call a class method?
To define a class method, just prefix the method with the @classmethod decorator and name the first parameter cls .
There are two ways to call:
- Call directly through the class name:
MyClass.show_class() - Called via instance:
my_instance.show_class()
The effects of the two methods are the same. It is recommended to call with class name, so that this is a class method more clearly.
In which scenarios are the class methods suitable for use?
Class methods are usually used in the following situations:
- Create factory methods to generate different instances of classes
- Modify the status of the class to affect all instances
- Encapsulates operations related to class but do not require instance data
For example, when you want to create an object according to different data formats, you can use class methods to encapsulate:
class Person:
def __init__(self, name):
self.name = name
@classmethod
def from_full_name(cls, full_name):
name = full_name.split(" ")[0]
return cls(name)This way you can use it like this:
p = Person.from_full_name("John Doe")
print(p.name) # output JohnCommon misunderstandings and precautions
- ? Do not access instance properties in class methods, because there is no
self - ? You can access class attributes, or use them to create instances
- ? If you use class methods in the inheritance system, the first parameter passed in will be the caller's class, not the current class
for example:
class A:
@classmethod
def who_am_i(cls):
print(cls.__name__)
class B(A):
pass
B.who_am_i() # Output BThis shows that the class method can automatically identify the actual calling class during inheritance.
Basically that's it. Although the class method is simple, it is very useful when designing class structures and providing flexible interfaces. As long as you pay attention not to mix class methods and instance methods, there will generally be no problems.
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