When a method is not preceded by the @staticmethod or @classmethod decorators and does not contain any references to the class or instance (via keywords like cls or self), Python raises the Method could be a function error. This is not a critical error, but you should check the code in question in order to determine if this section of code really needs to be defined as a method of this class.

Anti-pattern

In the Rectangle class below the area method calculates the area of any rectangle given a width and a height.

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height
        self.area = width * height
    # should be preceded by @staticmethod here
    def area(width, height):
        return width * height

area causes the Method could be a function error because it is ambiguous. It does not reference the instance or class using the self or cls keywords and it is not preceded by the @staticmethod decorator.

Class method is not preceded by @classmethod decorator

In the Rectangle class below the print_class_name method prints the name of the class. Again, Python raises the Method could be a function error because the method does not reference any class members or methods and is not preceded by the @classmethod decorator.

Furthermore, the first argument of a class method must be a reference to the class itself.

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height
        self.area = width * height
    # should be preceded by @classmethod here
    # missing required first argument "cls"
    def print_class_name():
        print("class name: Rectangle")

Best practices

Add the @staticmethod decorator before the static method

All static methods must be preceded by the @staticmethod decorator.

class Rectangle:
    # clarifies that this is a static method and belongs here
    @staticmethod
    def area(width, height):
        return width * height

Add the @classmethod decorator before the class method

All class methods must be preceded by the @classmethod decorator. Furthermore, the first argument of any class method must be cls, which is a reference to the class itself.

class Rectangle:
    @classmethod
    def print_class_name(cls):
        # "class name: Rectangle"
        print("class name: {0}".format(cls))

References

  • PyLint - R0201, no-self-use