Why python understanding "self", "this" and "that"?

Question

I am new to Python with Java background, the concept of "self" in function confuses me. I understand first argument "self" mean the object itself, but I do not understand how Python make this work. I also know that I could use "this" or "that" or "somethingElse", and Python would still understanding I mean to use the object.

I copied some code from a reddit post:

class A():
    def __init__(self):
        self.value = ""

    def b(this):
        this.value = "b"

    def c(that):
        that.value = "c"

a = A()
print(a.value)

a.b()
print(a.value)
>>>"b"
a.c()
print(a.value)
>>>"c"

How do python knows I do not mean to use an object here in the first argument? For example I modified the above code a bit:

class A():
    def __init__(self):
        self.value = ""

    def b(this):
        this.value = "b"

    def c(that):
        that.value = "c"

    def somethingElse(someObjectIWantToPass):
        someObjectIWantToPass.value = "still referring A.value"

class B():
    def __init__(self):
        self.value = ""
a = A()
print(a.value)

a.b()
print(a.value)

a.c()
print(a.value)

a.somethingElse()

print(a.value)

b = B()

a.somethingElse(b)

print (b.value)

And it broke:

b
c
still referring A.value
Traceback (most recent call last):
  File "D:/Documents/test.py", line 32, in <module>
    a.somethingElse(b)
TypeError: somethingElse() takes 1 positional argument but 2 were given

*“How do python knows I do not mean to use an object here in the first argument?”* It doesn’t know what you mean. If you *mean* to pass some arbitrary object to `A.somethingElse`, `a.somethingElse(b)` is simply incorrect. If you wanted a method operating on the class, you would use the `@classmethod` decorator, accept `(cls, someObjectIWantToPass)` as parameters, and call `A.somethingElse(b)`. — Ry-, Apr 13 '18 at 03:08
Related / maybe a dup: [What is the purpose of self](https://stackoverflow.com/questions/2709821/what-is-the-purpose-of-self) — miradulo, Apr 13 '18 at 03:09
Because the first argument for instance methods is always the instance itself, your `someObjectIWantToPass` becomes `self` in this case. You should rewrite it to read `def somethingElse(self, someObjectIWantToPass):`. — Selcuk, Apr 13 '18 at 03:15
Use `self`, `this`,`that`, `theOther`, `george`, `bob` or `delores` - it doesn't matter. The first argument is the object, *regardless* of what you call it :-) — paxdiablo, Apr 13 '18 at 03:40

Adam Smith · Accepted Answer · 2018-04-13T03:13:04.787

A method's first argument is always¹ its instance. Calling it self is idiomatic in Python but that name is strictly convention.

class A():
    def some_method(me):  # not called `self`
        print(str(id(me))

a = A()
a.some_method()
print(id(a))

If you're trying to pass another arbitrary object in, it has to be the second argument.

class B():
    def another_method(self, other):
        print(id(other))

b = B()
b.another_method(a)
print(id(b))  # different!
print(id(a))  # the same.

¹ Not actually always. @classmethod decorated methods use cls as their first argument, and @staticmethod` decorated methods have nothing passed to its first argument by default.

class C():
    @classmethod
    def some_classmethod(cls, other, arguments):
        # first argument is not the instance, but
        # the class C itself.

    @staticmethod
    def something_related(other, arguments):
        # the first argument gets neither the instance
        # nor the class.

*always* unless decorated with *classmathod* or *staticmethod*? — Stephen Rauch, Apr 13 '18 at 03:10
@StephenRauch *always* for the purposes of this answer, but I'll write a footnote. — Adam Smith, Apr 13 '18 at 03:11

smac89 · Answer 2 · 2018-04-13T03:19:30.237

You are too focused on syntactic sugar. Just realize that the first parameter in a non static member function in python is the reference to the current object. Whether you want to call it this, that, foobar, poop, it doesn't matter. The first parameter of a member function is considered the reference to the object on which the method is called.

The use of self is just a universal way everyone has understood it and the way Python recommends - a convention if you may.

The same goes for **kwargs and *args. These are simply conventions that have permeated the Python ecosystem and everyone just uses it that way, but it doesn't mean you can't give them a different name.

Your last example broke because the function you are calling (A.something) does not take any parameters. This will make sense if you understood what I had said earlier about first parameter in non static member function being a reference to the object on which the method was called.

Why python understanding "self", "this" and "that"?

2 Answers2