How to instantiate a class in Python?
As in many programming languages, the Python classes start with the class keyword and continue with the name of the class.
class MyClassName:
passThe instance of the class is created by calling the name of the class. The instance is also referred to as an object.
class MyClassName:
pass
obj = MyClassName()Class constructor
The initialization of the class invokes __init__() method, which is used to handle input parameters. To illustrate, the Student class takes name and age parameters by using __init__ as shown below code.
class Student:
def __init__(self, name, age):
self.name = name
self.age = age
first_student = Student("Emre", 19)
second_student = Student("Ayse", 20)
print(F"Student names: {first_student.name}, {second_student.name}")The output of the above code:
Student names: Emre, AyseThe first parameter named self of the init method refers to the instance of the class. The keyword self is a mandatory parameter if the __init__ method is used. Conversely, the following input parameters (first_variable, second_variable, third_variable) are optional and may vary depending on the programmer’s design.
def __init__(self, first_variable, second_variable, third_variable):Instance methods and variables
The instance methods of a class should take self as the first parameter. Otherwise, an error will occur.
class Student:
def __init__(name):
passThe error message due to above code.
TypeError: Student.__init__() takes 1 positional argument but 2 were givenIf the value of the input parameter needs to be stored in the instance of the class, a variable name with self should be used. It is not mandatory to use the same name for the instance variable and input variable. To clarify, as shown below, the input parameter of the init method is name, and the instance parameter is student_name. If the instance variable is called (as in the print method), the student_name should be used. On the other hand, the name is used to instantiate the Student class, and constructing the instance name will never be used.
class Student:
def __init__(self, name, age):
self.student_name = name
self.student_age = age
registered_student = Student(
name = "Ceren",
age = 25
)
print(registered_student.student_name)
print(registered_student.student_age)Instantiation without a constructor
A class can be instantiated without using the __init__ method. In addition, instance variables self.name and self.age can be created inside any instance method as shown code below.
class Student:
def define_values(self, name, age):
self.name = name
self.age = age
student = Student()
student.define_values("John", 22)
print(student.name)
print(student.age)On the contrary, if the variable is called before defining (as inside the print method) an error will be prompted as shown code below.
class Student:
def define_student_name(self, name):
self.name = name
student = Student()
print(student.name)
student.define_student_name("Nazrin")Error due to the above code.
AttributeError: 'Student' object has no attribute 'name'Accessing instance members within instance methods
Instance variables or methods can be called inside another instance method. To illustrate, self.name and self.surname are called inside get_student_fullname method.
class Student:
def __init__(self, name, surname):
self.name = name
self.surname = surname
def get_student_fullname(self):
return self.name + " " + self.surname
student = Student("Aygun", "Kazimova")
print(student.get_student_fullname())Conclusion
This post contains a simple explanation of how to instantiate a class, and we can call or design classes in different ways.