Oct 12, 2020 Notes

IT 212 -- Oct 12, 2020

Review Exercises

What are the dunder methods that you know? How are they invoked? Ans:

Dunder
Method Invoking
Expression

__init__ Constructor call, e.g.,
p = Person("Alice", "F", 11)

__str__ str(p) or print(p)

__repr__ repr(p)

__lt__ p < q

__eq__ p == q
How do you make objects of a class sortable?
Ans: Supply an __lt__ method for the class, which defines an < operator.

Dunder Method	Invoking Expression
__init__	Constructor call, e.g., p = Person("Alice", "F", 11)
__str__	str(p) or print(p)
__repr__	repr(p)
__lt__	p < q
__eq__	p == q

Look again at Practice Problem 1 in the October 7 Notes. Add a < operator method to your Person class to make the list of Person objects sortable. You can either make the Person objects sort in alphabetical order or in ascending order by age. Ans:

# In class, we sorted by age:
def __lt__(self, other):
    return self.age < other.age
    
# To sort in alphabetic order by name:
def __lt__(self, other):
    return self.name < other.name

# In test3.py, to sort with either __lt__ method:
persons.sort( )
print(persons)

Define a < operator for the Card class, where c1 < c2 if c1.rank < c2.rank. Now sort the list of Card objects by rank that we created on October 5 in the practice problem. Ans:
```
#__lt__ method for Card class.
def __lt__(self, other):
    return self.rank < other.rank
```

Define a == operator for the Card class using the __eq__ operator. For two cards c1 and c2, c1 == c2 if their ranks are equal. Test your == operator.

# __eq__ method for Card class
def __eq__(self, other):
    return self.rank == other.rank
    
# Unit test script test2.py to test == operator:
import card
import unittest

class MyUnitTestClass(unittest.TestCase):
        
    def test_1(self):
        c = card.Card(12, 'H')  #<-- QH
        d = card.Card(12, 'S')  #<-- QS
        e = card.Card(14, 'C')  #<-- AC
        self.assertEqual(c == d, True)
        self.assertEqual(d == e, False)
        
if __name__ == '__main__':
    unittest.main( )

Invoking Instance Methods

Usually we invoke (call) an instance method like this:

# p is a Person object.
p.have_birthday( )

# q is a Pet object.
q.set_vaccinated(True)

Alternatively, although not used much in practice, we can write the previous method calls like this:
```
Person.have_birthday(p)

Pet.set_vaccinated(q, True)
```

This clarifies why the self parameter is needed in these method definitions:

def have_birthday(self):
    self.age += 1
    
def set_vaccinated(self, v):
    self.vaccinated = v

Project 3a

Try out the code fragment in Project 3a that eliminates the non-letters and non-digits from a string.
Also try out the code in Project 3a that obtains the current year.
Look at the PetsDict Example. This example will help you with Project 3a, 3b, and 3c.
In the Animal class of the PetsDict Example, for the __init__ method, validate the gender instance variable to make sure that it is 'F' or 'M'. If the value of the gender parameter is not 'F' or 'M', set self.gender to 'O' for other.

Remember that test1.py and test2.py must test all public instance variables and all methods (public and private). Ans:
Here are the parts of the test1.py, test2.py, and test3.py modules that are relevant for Animal class:

=== animal.py ================================
# Add this method to the Animal class obtained
# from the PetsDict Example:
def __repr__(self):
    return f'<{str(self)}>'

=== test1.py =================================
from animal import Animal
a = Animal("dog", "German Shephard", "M")
print(a)
print(a.animal_type, a.breed, a.gender)

=== test2.py =================================
from animal import Animal
import unittest

class MyUnitTestClass(unittest.TestCase):

    def test_animal(self):
        a = Animal("dog", "German Shephard", "M")
        self.assertEqual(str(a), "dog German Shephard M")
        self.assertEqual(a.animal_type, "dog")
        self.assertEqual(a.breed, "German Shephard")
        self.assertEqual(a.gender, "M")

if __name__ == '__main__':
    unittest.main( )

=== test3.py =================================
from animal import Animal
animals = [ ]
fin = open("animals.txt", "r")
# Throw away first line
fin.readline( )
for line in fin:
    fields = line.split(',')
    a = Animal(fields[0], fields[1], fields[2])
    animals.append(a)

print(animals)

Introduction to Inheritance

A class B inherits a class A if a has all the instance variables and methods of the class A.
We use inheritance when we want to add functionality to an existing class, but we don't want to modify the existing class.
We write A ← B to mean B inherits from A.
An object of class B is also an object of class A, but with extra capabilities.
In Python, the following code defines the class B, which inherits from the class A:
```
class B(A):
   ... Put class definition for B here.
```
We already have used inheritance when we created unit test classes:
```
class MyUnitTestClass(unittest.TestCase):
```
Practice Problem: Design, implement, and test this inheritance hierarchy:
Person ← Employee ← Executive
Ans: See the EmployeeHierarchy Example.
A way to check whether inheritance makes sense for two classes A and B is to use the is-a test.
A employee is-a person (but with extra capabilities).
An executive is-a employee (but with extra capabilities).
Terminology for the classes A and B, where B inherits from A (B → A):

Class A Class B

Base Class Derived Class

Superclass Subclass

Parent Class Child Class

Class A	Class B
Base Class	Derived Class
Superclass	Subclass
Parent Class	Child Class

Project 3b

We will look at Project 3b later.
Look at the Project 3b Description.
Look at the Pet class and the test scripts (test1.py, test2.py) in the PetsDict Example.