[Python] Understand the self of the class. Learn the role of self from the execution result with or without self.

Anyone who started python wondered, Content that is full of content everywhere.

Reorganize for yourself.

### ■ Summary -It is necessary to set one argument of the method in the class by default. └ Otherwise an error └ Use self habitually

・ Self is optional └ Works even if it is not self

-When using a variable in the class, use the self. Variable. └ If you use x = 123, "self.x"

・ Global variables can be used as they are in the class

### ■ Verification details

-[Difference in behavior with and without self](Difference in behavior with and without #self) ・ [It doesn't have to be self](It doesn't have to be #self) -[Differences inside and outside the def class](Differences inside and outside the #def class) -[Set arguments with methods in class](#Set arguments with methods in class) -[Use variables in class](# Use variables in class)

## Difference in operation with and without self Create the following class to see the difference in operation with and without self.

** ▼ Confirmation class **

`class1`


class class1:
    def method1(self):
        print("Executed method1")

x = class1()
x.method1()

#output
#Executed method1

** ▼ Without self **

`No self`


class class1:
    def method1():
        print("Executed method1")

x = class1()
x.method1()

#output
# TypeError: method1() takes 0 positional arguments but 1 was given

** ■ Error ** method1() takes 0 positional arguments but 1 was given

The method "method1" defined in the class does not take an argument (0), but 1 is given.

Even in the executed process, the inside of () is empty and no argument is entered. (Normally argument 0) x.method1()

** ■ Cause ** When executing a method in a class, the instance itself is automatically given as an argument.

Example
x.method1 () → Argument 1 x.method1 ('A','B') → Argument 3

** ■ Countermeasure ** The methods in the class must be given at least one argument by default.

⇛ Its argument is "self"

The argument may be other than self. It is customary to use self.

## It doesn't have to be self ** ▼ When using characters other than self **

When the argument self is replaced with "a" and "aiueo".

`Characters other than self ①`


class class1:
    def method1(a):
        print("Executed method1")

x = class1()
x.method1()

#output
#Executed method1

`Characters other than self ②`


class class1:
    def method1(AIUEO):
        print("Executed method1")

x = class1()
x.method1()

#output
#Executed method1

Both run without problems.

## Differences inside and outside the def class

If you bring the defined method out of the class, of course no arguments are passed by default.

The function defined by def in the class is called a method.
Functions outside the class.

`Out of class`


def method1():
    print("Executed method1")
    
method1()

#output
#Executed method1

The opposite of the method in the class, if you pass only one argument, you will get an error.

`error`


def method1():
    print("Executed method1")
    
method1('A')

#output
# TypeError: method1() takes 0 positional arguments but 1 was given

Error: method1 () takes 0 positional arguments but 1 was given

An error that there are 0 arguments but 1 extra.

## Set arguments with methods in the class

Since one argument is set by default in the class, when setting other arguments, it is necessary to set one more than usual. (Becomes self + α)

`When setting arguments`


class class2:
    def weather(self, day, sky):
        print(day + 'The weather is' + sky + 'is.')

x = class2()
x.weather('today', 'Sunny')

#output
#The weather is sunny today.

Set three arguments of the weather method in the class: "self", "day", and "sky".

** ▼ When "self" is changed to "a" ** Of course, it works normally even if it is not "self".

`Other than self`


class class2:
    def weather(a, day, sky):
        print(day + 'The weather is' + sky + 'is.')

x = class2()
x.weather('today', 'Sunny')

#output
#The weather is sunny today.

## Use variables in the class

The big advantage of a class is that you can set functions and variables that are valid only within the class. (Not global)

You need to combine it with "self" to use variables in the class.

** ▼ Use variables in the class **

To call the variable "x = 123" defined in the class in the method, it must be "self.x".

`class3`


class class3:
    x = 123

    def method1(self):
        print(self.x)

cls = class3()
cls.method1()  

#output
# 123

** ▼ If you do not add self **

If there is no self in "self.x", an error will occur.

`x only`


class class3:
    x = 123

    def method1(self):
        print(x)

cls = class3()
cls.method1()  

#output
# NameError: name 'x' is not defined

Error: name'x' is not defined "X = 123" in the class is not "x".

** ▼ Use global variables **

If you use a variable defined outside of class, you don't need self.

`Use global variables`


x = 123

class class3:

    def method1(self):
        print(x)

cls = class3()
cls.method1()

#output
# 123

By using self, you can clearly distinguish whether it is a global variable or a variable in a class.

** ▼ Other than self ** By the way, it works other than self.

`Other than self`


class class3:
    x = 123
    
    def method1(AIUEO):
        print(AIUEO.x)

cls = class3()
cls.method1()  

#output
# 123

I was wondering what self is, but I think it's convenient and necessary to know how to use it.