What is an "instance method"?

>>> class Foo:
...     def im_a_method(self):
...         pass
... 
>>> x = Foo()
>>> x.im_a_method
<bound method Foo.im_a_method of <__main__.Foo object at 0x7f4f1993dd30>>

Tada! That’s an instance method object. It’s the thing you get when you retrieve a method of an object, before you call it.

What is an instance method?

An instance method is a function the is bound to a class instance. The instance of the class is implicitly passed as the first argument to instance methods. It essentially belongs to that specific instance. An instance method is the “normal” type of method people use. This is opposed to a static method or class method created using staticmethod and classmethod respectively.

Here’s an example of an instance method:

>>> class Class: 
...     def method(self): 
...        pass 

>>> Class.method
<bound method Class.method of <Class object at 0x7f12781c5b70>>

It’s that simple.

An instance method:

• can call instance and class variables and instance, class and static methods by self.

• can call class variables and instance, class and static methods by class name but not instance variables.

• can be called by object.

• can be also called directly by class name but when called directly by class name, we need to pass one argument to the instance method because self becomes the normal parameter which doesn’t have the ability to call instance and class variables and instance, class and static methods.

• needs self for the 1st argument otherwise the instance method cannot be called by an object but the instance method can still be called directly by class name and the name of self is used in convention so other names instead of self still work.

*I also explain about @classmethod and @staticmethod in my answer for @classmethod vs @staticmethod in Python.

For example, the instance method can call the instance and class variables and the instance, class and static methods by self and the instance method can call the class variable and the instance, class and static methods by class name but not the instance variables and the instance method can be called by object as shown below:

class Person:
    x = "Hello"
    def __init__(self, name):
        self.name = name
    
    def test1(self): # Instance method
        print(self.name) # Instance variable by "self"
        print(self.x)    # Class variable by "self" 
        self.test2()     # Instance method by "self"
        self.test3()     # Class method by "self"
        self.test4()     # Static method by "self"
       
        print()

        print(Person.x)       # Class variable by class name
        Person.test2("Test2") # Instance method by class name
        Person.test3()        # Class method by class name
        Person.test4()        # Static method by class name

    def test2(self):
        print("Test2")
        
    @classmethod
    def test3(cls):
        print("Test3")
        
    @staticmethod
    def test4():
        print("Test4")

obj = Person("John")
obj.test1() # By object

Output:

John  # Instance variable by "self"
Hello # Class variable by "self"
Test2 # Instance method by "self"
Test3 # Class method by "self"
Test4 # Static method by "self"

Hello # Class variable by class name
Test2 # Instance method by class name
Test3 # Class method by class name
Test4 # Static method by class name

And, if the instance method tries to call the instance variable by class name as shown below:

# ...
    
    def test1(self): # Instance method
        print(Person.name) # Instance variable by class name

obj = Person("John")
obj.test1()

The error below occurs:

AttributeError: type object ‘Person’ has no attribute ‘name’

And, the instance method can be also called directly by class name but when called directly by class name, we need to pass one argument to the instance method as shown below because self becomes the normal parameter which doesn’t have the ability to call the instance and class variables and the instance, class and static methods by self:

# ...

    def test1(self): # Instance method
        print(self)

# ...

Person.test1("Test1") # Here

Output:

Test1

So, if the instance method tries to call the instance and class variables and the instance, class and static methods by self as shown below:

# ...

    def test1(self): # Instance method
        print(self.name) # Instance variable or
        print(self.x)    # Class variable or
        self.test2()     # Instance method or
        self.test3()     # Class method or
        self.test4()     # Static method

# ...

Person.test1("Test1") # Here

The errors below occur because again, self becomes the normal parameter which doesn’t have the ability to call the instance and class variables and the instance, class and static methods:

AttributeError: ‘str’ object has no attribute ‘name’

AttributeError: ‘str’ object has no attribute ‘x’

AttributeError: ‘str’ object has no attribute ‘test2’

AttributeError: ‘str’ object has no attribute ‘test3’

AttributeError: ‘str’ object has no attribute ‘test4’

And, if one argument is not passed to the instance method as shown below:

# ...

    def test1(self): # Instance method
        print(self)

# ...

Person.test1() # Here

The error below occurs:

TypeError: test1() missing 1 required positional argument: ‘self’

And, the instance method needs self for the 1st argument otherwise the instance method cannot be called by object as shown below:

# ...

    def test1(): # Without "self"
        print("Test1")

# ...

obj = Person("John")
obj.test1() # Here

Then, the error below occurs:

TypeError: test1() takes 0 positional arguments but 1 was given

But, the instance method without self can still be called directly by class name as shown below:

# ...

    def test1(): # Without "self"
        print("Test1")

# ...

Person.test1() # Here

Output:

Test1

And, the name of self is used in convention in an instance method so other name instead of self still works as shown below:

# ...
             # Here
    def test1(orange): # Instance method
        print(orange.name) # Instance variable
        print(orange.x)    # Class variable
        orange.test2()     # Instance method
        orange.test3()     # Class method
        orange.test4()     # Static method

# ...

obj = Person("John")
obj.test1()

Output:

John
Hello
Test2
Test3
Test4