A constructor is a special method (function) definition within a class. The constructor method is executed when an object is created, that is if the class has a constructor. If a constructor is not defined, it is automatically defined to be an empty function. In Python, it is only possible to have one constructor, which uses the special name –
and is defined under class variables in the class definition. In Python, we are only allowed to have one constructor, since function overloading is not used in the language. We can define a function with the same name multiple times but Python will use the function which was defined last, we can see it as overriding the previous function.
Let us define a
Cat class, which has a tail. The number of legs, fur color and a race are added when an object is created.
class Cat: tail = True def __init__(self, numLegs, colFur, race): self.numberOfLegs = numLegs self.colorOfFur = colFur self.race = race
In this example we are forcing the user to enter information into the class by defining
__init__ to expect 3 parameters.
bob = Cat(4, "gray", "siamese") // Works print(bob.tail) // True print(bob.colorOfFur) // gray bob = Cat() // Does not work
Let us define the cat class in a different way to not recieve initial information.
class Cat: tail = true def __init__(self): self.numberOfLegs = 4 self.colorOfFur = "gray" self.race = "siamese"
Now the constructor is empty and no parameters are needed, the
self parameter is always defined since that makes a reference to the object.
bob = Cat() // Works now! bob = Cat(4, "gray", "siamese") // Does not work now!