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
