We’ll now look at a special function that is part of most classes: the Python constructor. A constructor is a function that is called automatically when an object is created. A constructor can optionally accept arguments as well, just like a regular function.
Table of contents
The default constructor
When creating an object from a class, it looks like we are calling a function:
car = Car()
Well… it doesn’t just look like we are calling a function, we are in fact calling a function! This method, which we did not have to define, is called the constructor. It constructs and initializes the object. Every class has one by default, called
__init__, even if we don’t define it ourselves. This has to do with inheritance, which you’ll learn about shortly.
Have you ever used the
str() function to convert an object into a class? Or perhaps the
int() function to convert a string into a number?
>>> 'a' + str(1) 'a1' >>> int('2') + 2 4
What you are actually doing here, is creating new objects of type
int by calling the constructors of the classes
Creating your own Python constructor
We can override the
__init__ method, to give it extra abilities by accepting arguments. Let’s redefine the
Car class using a custom constructor:
class Car: def __init__(self, started = False, speed = 0): self.started = started self.speed = speed def start(self): self.started = True print("Car started, let's ride!") def increase_speed(self, delta): if self.started: self.speed = self.speed + delta print("Vrooooom!") else: print("You need to start the car first") def stop(self): self.speed = 0
Our custom Python constructor has named parameters with default values, so we can create instances of the class
Car in multiple ways:
>>> c1 = Car() >>> c2 = Car(True) >>> c3 = Car(True, 50) >>> c4 = Car(started=True, speed=40)
You may have noticed a flaw: we can now create a new car that is not started but set its speed anyway. For now, let’s leave it at that.