In the last post, I discussed about variables - one of the two basic building blocks of a class. In this post, we will go over the other building block - methods. In case you need a refresher on variables you can take a look at my previous post - Object Oriented Programming in Python - Understanding Variables.
A Primer on Methods¶
A method is a function embedded inside a class.
Method must have at least one parameter. There is no such thing as parameterless method. Typically self is used as a parameter in methods that don't need any custom parameter. self refers to the object for which the method is called and makes the object's variables and methods available for the method. Using the word self is a norm that is typically followed. So even though self can be replaced with any other word, doing so will add unnecessary surprises.
Case¶
Let's assume that from our successful delivery of the first simple project on creating a list with the ability to add and remove elements, we have got a new customer. He wants a similar list but with a slightly strict requirement - he wants to restrict the type of elements only to numeric types.
Since the basic requirements are still the same:
- It's a list,
- Should be able to add and remove elements,
- And show the entire list,
We will re-use the class NumList that we created earlier and build on that using a subclass called StrictNumList. Inside StrictNumList we can see two methods have been defined: __check_value(), and add_value(). More about this is provided later.
class NumList:
def __init__(self, name = ""):
self.instName = name
self.__list = []
def add_value(self, val):
self.__list.append(val)
def remove_value(self):
rv = self.__list[-1]
del self.__list[-1]
return rv
def get_list(self):
return self.__list
class StrictNumList(NumList):
def __init__(self, name):
super().__init__(name)
def __check_value(self, val):
return str(val).isalpha()
def add_value(self, val):
if not self.__check_value(val):
super().add_value(val)
else:
input_val = input("Insert a number: ")
val = float(input_val)
super().add_value(val)
sl01 = StrictNumList(name = "Number List B1")
sl01.instName
sl01.add_value(2)
sl01.add_value("abc")
Insert a number: 5
sl01.get_list()
[2, 5.0]
Constructor - A Special Method¶
__init__ or Python constructor is a special kind of method that's used to initiate a class.
- It runs every time the class is instantiated and makes the class usable.
- It must have at least one parameter -
selfto make properties of the object available. - Unlike other methods,
__init__can't return a value or can't be invoked inside or outside of a class. For example, you can't create call a method like <object name><.>__init__().
Types of Methods¶
Like variables, methods can be public or private, or better called partially private (remember name mangling?).
Private Methods¶
- Naming a private method follows the same pattern as the private variables - add two underscores (
__) before the method name. - Name mangling works with class name the same way it works with the variables.
Why Use a Private Method?
The same reason for using a private variable. The primary reason for creating these methods is to be used in other methods thus they don't need to be accessible directly from objects.
But as we know from the last blog, name mangling actually makes them available. So using the mangled name we can actually access the private methods too.
In our example class StrictNumList we have created __check_value() as a private method. Because it's the main purpose is to be used inside the add_value() method. But chec kout the below code to see how we can still access this method using the mangled name!
# calling private method - __check_value() using its mangled name
sl01._StrictNumList__check_value('5.5')
False
Public Methods¶
Public methods can be accessed directly from the object using dot notation i.e. <object name><.><method name>. In our example, add_value() is a public variable.
🛑 But notice how we have defined this method in both the super and subclass. This is called method overriding.
Method overriding happens when you define the same method differently in the subclass.
To use method overriding successfully you need to meet two conditions:
- It's only possible in the class inheritance context. Or in other words, method inheritance doesn't work inside the same class.
- The subclass should have the same name and number of parameters as the superclass.
🛑 Also notice, how the add_value() method from the superclass was utilized in the add_value() method in subclass SuperNumList.
What's Next¶
Till now in this OOP in Python series,
- We have got introduced to classes and their basic building blocks,
- We have learned about inheritance and its application of it using Subclass,
- We have got detailed into variables and in this post methods.
In the next post, we will try to get an insider view of the Python exceptions. And in doing so we will see how learning about these topics will come in handy! Happy coding!