Python - why I must use self in this method?

Question

class ValidateStrategy(abc.ABC):
    def validate(self, input_string):
        pass


class IntValidateStrategy(ValidateStrategy):
    def validate(self, input_string):
        try:
            int(input_string)
            return True
        except ValueError:
            return False


def string_validate(input_string, validate_strategy):
    return validate_strategy.validate(input_string)

string_validate("22", IntValidateStrategy)

Why am I getting an error because the self argument is missing?

> Traceback (most recent call last):   File "F:\Pobrane\drive\test.py",
> line 11, in <module>
>     print(el + ": " + str(validator.string_validate(el, validator.IntValidateStrategy)))   File
> "F:\Pobrane\drive\validator.py", line 34, in string_validate
>     return validate_strategy.validate(input_string) TypeError: IntValidateStrategy.validate() missing 1 required positional argument:
> 'input_string'

You passed in the class itself into validator.string_validate(), did you meen to instead pass in an instance of IntValidateStrategy? e.g. validator.string_validate("22", validator,IntValidateStrategy())? Either that or you can make the validate function into a static function, allowing you to call it on the class without passing in a "self" isntance. — Scotty Jamison
– Scotty Jamison, Commented Apr 5, 2022 at 22:07

RossM · Accepted Answer · 2022-04-05 22:27:50Z

4

Short answer, you don't need to.

Whenever you call a method of a class, unless it is decorated as a @classmethod or @staticmethod the first argument will be the instance of the class the method was called from.

To explain:

If I call the method x as Class.x(), then no first argument will be automatically passed.

If I call the method x as instance.x() the first argument will be the instance.

You are passing IntValidateStrategy to your function. This means that the first argument in the function will be the class IntValidateStrategy.

Since you aren't accessing any variables within the class, you can add the decorator @staticmethod above your method definition, and remove the self argument since nothing is being passed automatically.

This means when you call IntValidateStrategy.validate(...), nothing is passed automatically and you are free to use your function.

Code:

class IntValidateStrategy(ValidateStrategy):
    @staticmethod
    def validate(input_string):
        try:
            int(input_string)
            return True
        except ValueError:
            return False

answered Apr 5, 2022 at 22:27

RossM

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4 Comments

Brad Solomon Over a year ago

One key clarification for second point: the first argument will implicitly be the instance (instance). This trips a lot of people up!

ImLearning Over a year ago

But why when I don't use them as static do I have such an error? In my opinion, it should not be present. In theory, I know I can convert it to static, but I'm interested in this particular case without staticmethod. The advice about staticmethod is correct, but I am well aware of this solution. I just want to know what happened here that makes the compiler react to the given code.

RossM Over a year ago

When you don't have them as static, they are automatically expecting the first argument to be an instance the instance or class the method was called from. I would reccomend this video on static, class and instance methods

ImLearning Over a year ago

I know how self works, but I haven't noticed one detail. When we pass a function, it's without the parentheses. But I don't pass a function there, it's just an object instance constructor. In a word, it was a Czech mistake, probably caused by fatigue. Generally, your comments were valuable, but it was about something completely different, I just got it wrong. I gave the validate_strategy argument as: IntValidateStrategy instead of DateValidateStrategy ()!

Chris · Accepted Answer · 2022-04-05 22:34:02Z

Functions are non-data descriptors in Python.

When you access a function through a class, you get the function exactly as you defined it. As a concrete example, IntValidateStrategy.validate is a function that accepts two named positional arguments, self, and string.

Descriptor magic is invoked when you access a function through an instance of a class. Descriptor protocol means calling validate.__get__(instance). For regular functions, this returns a bound method object, which has a parameter list that does not explicitly include the first positional parameter of the function you defined, self. A concrete example would be if you instantiated your strategy:

IntValidateStrategy().validate('234')

The simplest fix for your example would be to instantiate the class you are passing in:

string_validate("22", IntValidateStrategy())

A better solution might be too allow string_validate to accept any callable as a validator:

def string_validate(input_string, validate_strategy):
    return validate_strategy(input_string)

string_validate("22", IntValidateStrategy().validate)

At this point, you don't need to even have string_validate. In fact you can simplify your interface and make it more flexible by replacing the name validate with __call__. You'll be able to use functions for stateless validators, and full classes if you need state:

class ValidateStrategy(abc.ABC):
    def __call__(self, input_string):
        pass

def stateless_int_validate_strategy(input_sting):
    try:
        int(input_string)
        return True
    except ValueError:
        return False

class StatefulIntValidateStrategy(ValidateStrategy):
    def __call__(self, input_string):
        return stateless_int_validate_strategy(input_sting)

stateless_int_validate_strategy("22")
validator = StatefulIntValidateStrategy()
validator("23")

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