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Method has no argument

Unlike some programming languages, Python does not pass references to instance or class objects automatically behind the scenes. So the program must explicitly pass them as arguments whenever it wants to access any members of the instance or class within a method.

Anti-pattern

In the Rectangle class below the area method attempts to return the value of the area instance variable. However, self.area is undefined because a reference to the instance object has not been explicitly passed as an argument to the method.

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height
        self.area = width * height
    # missing first argument "self"
    def area():
        # self is undefined here
        return self.area

Class method is missing the cls keyword

The method print_class_name attempts to print the name of the class. However, to programmatically access a class name, a method needs to have a reference to the class object. This is accomplished by passing the keyword cls as the first argument to the method. Because print_class_name does not do this, its reference to cls in the body of the method is undefined.

class Rectangle:
    @classmethod
    # missing first argument "cls"
    def print_class_name():
        # cls is undefined here
        print("Hello, I am {0}!".format(cls))

The method area computes the value of any rectangle. Currently this method is ambiguous. It is defined as a method of the Rectangle class, yet it does not reference any instance or class members. The method needs to explicitly state that it is a static method via the @staticmethod decorator.

class Rectangle:
    # "@staticmethod" should be here
    def area(width, height):
        return width * height

Best practices

Add the self parameter to instance methods

To access the area member of a Rectangle instance the first argument of the area method needs to be a reference to the instance object, signified by the keyword self.

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height
        self.area = width * height
    # instance members now accessible because of "self"
    def area(self):
        return self.area

Add the cls parameter to class methods

To access the name of the class the print_class_name method needs to explicitly pass an argument to the class object. This is done by adding the keyword cls as the first argument of the method.

class Rectangle:
    @classmethod
    # class members now accessible, thanks to "cls"
    def print_class_name(cls):
        print("Hello, I am {0}!".format(cls))

Add the @staticmethod decorator to static methods

If the method is a static method that does not need access to any instance members, then the method should be preceded by the @staticmethod decorator. This improves readability by helping clarify that the method should never rely on any instance members.

class Rectangle:
    # clarifies that the method does not need any instance members
    @staticmethod
    def area(width, height):
        return width * height

References

  • PyLint - E0211, no-method-argument

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