python - Difference between staticmethod and classmethod

ID : 88

viewed : 289

Tags : pythonoopmethodspython-decoratorspython

Top 5 Answer for python - Difference between staticmethod and classmethod

vote vote

92

Maybe a bit of example code will help: Notice the difference in the call signatures of foo, class_foo and static_foo:

class A(object):     def foo(self, x):         print(f"executing foo({self}, {x})")      @classmethod     def class_foo(cls, x):         print(f"executing class_foo({cls}, {x})")      @staticmethod     def static_foo(x):         print(f"executing static_foo({x})")  a = A() 

Below is the usual way an object instance calls a method. The object instance, a, is implicitly passed as the first argument.

a.foo(1) # executing foo(<__main__.A object at 0xb7dbef0c>, 1) 

With classmethods, the class of the object instance is implicitly passed as the first argument instead of self.

a.class_foo(1) # executing class_foo(<class '__main__.A'>, 1) 

You can also call class_foo using the class. In fact, if you define something to be a classmethod, it is probably because you intend to call it from the class rather than from a class instance. A.foo(1) would have raised a TypeError, but A.class_foo(1) works just fine:

A.class_foo(1) # executing class_foo(<class '__main__.A'>, 1) 

One use people have found for class methods is to create inheritable alternative constructors.


With staticmethods, neither self (the object instance) nor cls (the class) is implicitly passed as the first argument. They behave like plain functions except that you can call them from an instance or the class:

a.static_foo(1) # executing static_foo(1)  A.static_foo('hi') # executing static_foo(hi) 

Staticmethods are used to group functions which have some logical connection with a class to the class.


foo is just a function, but when you call a.foo you don't just get the function, you get a "partially applied" version of the function with the object instance a bound as the first argument to the function. foo expects 2 arguments, while a.foo only expects 1 argument.

a is bound to foo. That is what is meant by the term "bound" below:

print(a.foo) # <bound method A.foo of <__main__.A object at 0xb7d52f0c>> 

With a.class_foo, a is not bound to class_foo, rather the class A is bound to class_foo.

print(a.class_foo) # <bound method type.class_foo of <class '__main__.A'>> 

Here, with a staticmethod, even though it is a method, a.static_foo just returns a good 'ole function with no arguments bound. static_foo expects 1 argument, and a.static_foo expects 1 argument too.

print(a.static_foo) # <function static_foo at 0xb7d479cc> 

And of course the same thing happens when you call static_foo with the class A instead.

print(A.static_foo) # <function static_foo at 0xb7d479cc> 
vote vote

84

A staticmethod is a method that knows nothing about the class or instance it was called on. It just gets the arguments that were passed, no implicit first argument. It is basically useless in Python -- you can just use a module function instead of a staticmethod.

A classmethod, on the other hand, is a method that gets passed the class it was called on, or the class of the instance it was called on, as first argument. This is useful when you want the method to be a factory for the class: since it gets the actual class it was called on as first argument, you can always instantiate the right class, even when subclasses are involved. Observe for instance how dict.fromkeys(), a classmethod, returns an instance of the subclass when called on a subclass:

>>> class DictSubclass(dict): ...     def __repr__(self): ...         return "DictSubclass" ...  >>> dict.fromkeys("abc") {'a': None, 'c': None, 'b': None} >>> DictSubclass.fromkeys("abc") DictSubclass >>>  
vote vote

72

Basically @classmethod makes a method whose first argument is the class it's called from (rather than the class instance), @staticmethod does not have any implicit arguments.

vote vote

69

Official python docs:

@classmethod

A class method receives the class as implicit first argument, just like an instance method receives the instance. To declare a class method, use this idiom:

class C:     @classmethod     def f(cls, arg1, arg2, ...): ...  

The @classmethod form is a function decorator – see the description of function definitions in Function definitions for details.

It can be called either on the class (such as C.f()) or on an instance (such as C().f()). The instance is ignored except for its class. If a class method is called for a derived class, the derived class object is passed as the implied first argument.

Class methods are different than C++ or Java static methods. If you want those, see staticmethod() in this section.

@staticmethod

A static method does not receive an implicit first argument. To declare a static method, use this idiom:

class C:     @staticmethod     def f(arg1, arg2, ...): ...  

The @staticmethod form is a function decorator – see the description of function definitions in Function definitions for details.

It can be called either on the class (such as C.f()) or on an instance (such as C().f()). The instance is ignored except for its class.

Static methods in Python are similar to those found in Java or C++. For a more advanced concept, see classmethod() in this section.

vote vote

53

To decide whether to use @staticmethod or @classmethod you have to look inside your method. If your method accesses other variables/methods in your class then use @classmethod. On the other hand, if your method does not touches any other parts of the class then use @staticmethod.

class Apple:      _counter = 0      @staticmethod     def about_apple():         print('Apple is good for you.')          # note you can still access other member of the class         # but you have to use the class instance          # which is not very nice, because you have repeat yourself         #          # For example:         # @staticmethod         #    print('Number of apples have been juiced: %s' % Apple._counter)         #         # @classmethod         #    print('Number of apples have been juiced: %s' % cls._counter)         #         #    @classmethod is especially useful when you move your function to another class,         #       you don't have to rename the referenced class       @classmethod     def make_apple_juice(cls, number_of_apples):         print('Making juice:')         for i in range(number_of_apples):             cls._juice_this(i)      @classmethod     def _juice_this(cls, apple):         print('Juicing apple %d...' % apple)         cls._counter += 1 

Top 3 video Explaining python - Difference between staticmethod and classmethod

Related QUESTION?