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The code below shows that when working with different object types like, int, str or list, the + operator performs different functions. This is known as operator overloading and is another form of polymorphism.

# For an int and an int, + returns an int 2 + 4 == 6 # For a string and a string, + returns a string "Is this " + "addition?" == "Is this addition?" # For a list and a list, + returns a list [1, 2] + [3, 4] == [1, 2, 3, 4]

To implement this behavior, we must first discuss dunder methods. Every defined class in Python has access to a group of these special methods. We’ve explored a few already, the constructor __init__() and the string representation method __repr__(). The name dunder method is derived from the Double UNDERscores that surround the name of each method.

Defining a class’s dunder methods is a way to perform operator overloading.

class Animal: def __init__(self, name): self.name = name def __add__(self, another_animal): return Animal(self.name + another_animal.name) a1 = Animal("Horse") a2 = Animal("Penguin") a3 = a1 + a2 print(a1) # Horse print(a2) # Penguin print(a3) # HorsePenguin

The above code has the class Animal with a dunder method, .__add__(). This defines the + operator behavior when used on objects of this class type. The method returns a new Animal object with the names of the operand objects concatenated. In this example, we have created a "HorsePenguin"!

The line of code a3 = a1 + a2 invokes the .__add__() method of the left operand, a1, with the right operand a2 passed as an argument. The name attributes of a1 and a2 are concatenated using the .__add__() parameters, self and another_animal. The resulting string is used as the name of a new Animal object which is returned to become the value of a3.

Instructions

1.

There is now a Meeting class with an attendees list attribute and an .__add__() dunder method that adds Employee instances to the attendees list. Before we try and add employees to a meeting, we want to make sure we can know how many employees are in a meeting.

Inside the Meeting class:

  • Overload the len() operation by defining a __len__() dunder method
  • Inside the __len__() definition, return the length of the attribute attendees
2.

Now add three employees to a meeting:

  • Using the Meeting instance m1, add each of the employee instances e1, e2, and e3. Use one line for each employee instance.
  • Output the length of meeting instance m1

You should see the output from each employee being added and then the length of the meeting, 3.

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