Python Classes

Intro to Classes

Objectives

• Understand difference between objects and classes

• Understand how classes are defined

• Understand how objects are initialized

• Understand instance variables and instance methods

• Understand class variables and class methods

• Utilize the self keyword

• Understand method chaining in a class

Resources

Fork or clone this repo to gain access to a complete working end-product of the examples in this lesson:

https://github.com/SEI-ATL/python-class-examples

Classes

Python is an object oriented language. Object oriented languages allow us to create things that act like physical objects in our day-to-day lives. Every day we interact with objects like chairs, beverages, and CDs. These objects have properties that define them, and they have things we can do with them.

If I were to describe the properties that make up coffee I drink I would say each cup of coffee I drink has a capacity, a current amount. There's also things I an do with each cup of coffee I drink. I can fill my cup. I can empty my cup. I can drink some of my coffee. Python classes allow us to model the properties of my coffee and define how I can interact with my coffee.

I'm not the only person who drinks coffee. A good class definition models what a cup of coffee is for everyone. Then, we can use the class definition to create instances of the class so I can my coffee, Sean can have his coffee and Brandi can have her Coffee. Each instance of the coffee class can have a different capacity, and keep track of different amounts. Although our coffees have different properties the properties are affected by actions like fill, empty, and drink similarly.

Here's what a Coffee class would look like in Python:

class CoffeeCup():
  def __init__(self, capacity):
    self.capacity = capacity
    self.amount = 0

  def fill(self):
    self.amount = self.capacity

  def empty(self):
    self.amount = 0

  def drink(self, amount):
    self.amount -= amount
    if (self.amount == 0):
      self.amount = 0

The CoffeeCup is a collection of variables and methods. The variables in this class are self.capacity and self.amount. The methods in this class are fill, empty, and drink. The __init__ method is a special method Python executes when a new cup of coffee is created.

The self keyword is similar to the this keyword in JavaScript. The self keyword allow each instance of a CoffeeCup to know what it's own capacity is and what it's own current amount is.

Create instances of a class by calling ClassName(). This invokes the __init__ method. You can pass parameters to it too, ClassName(param1, param2).

Here's how Steve, Sean and Brandi could each have their own cup of coffee. Let's assume the capacity and amount units are all in ounces.

steves_cup = CoffeeCup(12)  # a fancy latte.
seans_cup = CoffeeCup(16)    # gas station drip.
brandis_cup = CoffeeCup(2)  # a quick espresso.

Each of our cups start empty and have their own capacity. Let's fill the cups, have everyone take a 1 ounce drink, and print the amount left in each cup.

steves_cup.fill()
seans_cup.fill()
brandis_cup.fill()

steves_cup.drink(1)
seans_cup.drink(1)
brandis_cup.drink(1)

print(steves_cup.amount, "ounces left")
print(seans_cup.amount, "ounces left")
print(brandis_cup.amount, "ounces left")

That's the basics of how to create and interact with objects in Python!

Exercise: Create Your Own Class

Write a BankAccount class.

• Bank accounts should be created with the kind of account (like "savings" or "checking").

• Each account should keep track of it's current balance.

• Each account should have access to a deposit and a withdraw method.

• Each account should start with a balance set to zero.

• return the amount withdrawn, for convenience

Create a checking account and a savings account. Withdraw money from the savings account and deposit that amount into the checking account.

Bonus: start each account with an additional overdraft_fees property that starts at zero. If a call to withdraw ends with the balance below zero then overdraft_fees should be incremented by twenty.

class BankAccount():
  def __init__(self, kind):
    self.kind = kind
    self.balance = 0
    self.overdraft_fees = 0

  def deposit(self, amount):
    self.balance += amount

  def withdraw(self, amount):
    self.amount -= amount
    if (self.amount < 0):
      self.overdraft_fees += 20
    return amount

Default Parameters

Python allows us to provide default values for parameters in any function we provide. Let's write a Point class that has x and y variables. If no x and y values are provided when a Point is initialized x and y should both default to zero.

Add a method called distance that calculates and returns the distance between the current point and the origin. Use the mathematical distance formula where the distance between a point and the origin is defined as the square root of (x_x + y_y).

Use Python's exponent ** operator to calculate square root. 9 ** .5 == 3.0

p0 = Point()
p2 = Point(3, 4)

print(p0.distance())
0.0

print(p2.distance())
5.0

Printing Objects

Ever tried to have Python print an object? It's nasty. If you try to print an object Python will print a representation of the object where you'll see what type of an object it is and it will show you a number representing something about where the object exists in memory, which we don't care about.

print(p0)
<__main__.Point object at 0x107335630>

We can write a special method __str__ that Python will call when an object is printed or turned in to a string. Customizing this method in our classes makes our programs much easier to interact with.

Notice that Python goes out of it's way to improve the readability of code. Any method that looks like __init__ or __str__ with underscores has a special purpose in the language. Python uses the underscores to make it immediately clear that this is where the magic happens!

Let's define a __str__ method in our Point class that will print out points like we're used to seeing points.

• p0 in the example above should appear as "(0,0)"

print(p0) # (0,0)

• p2 in the example above should appear as "(3,4)".

print(p2) # (3,4)

class Point():
  def __init__(self, x=0, y=0):
    self.x = x
    self.y = y

  def __str__(self):
    return "({},{})".format(self.x, self.y)

  def distance(self):
    return (self.x ** 2 + self.y ** 2) ** .5

Now we can create points and when we print those objects we see something pretty printed instead of the garbly-goop we saw before.

p0 = Point()
p2 = Point(3, 4)

print(p0)
(0,0)

print(p2)
(3,4)

Class Variables

In our CoffeeCup example and the BankAccount example and in our Point example each class has variables attached to the self property that exist independently for each object that's created. We can also attach variables to the class itself so that there's one single thing that exists for an entire class. These are called class variables.

For the Point class we'll create a class variable to represent ORIGIN. Class variables are available even without creating any instances of a class. We'll be able to write code that references Point.ORIGIN by itself.

Change the distance method to accept a reference to a second Point as an optional parameter. The second point parameter should have a default value of None. We will write an if statement to detect when p2 is None and set it to Point.ORIGIN instead.

It's hard to reference the Point class in the class definition itself because it hasn't finished being created yet. We'll attach ORIGIN to the Point class after it's defined.

class Point():
  def __init__(self, x=0, y=0):
    self.x = x
    self.y = y

  def __str__(self):
    return "({},{})".format(self.x, self.y)

  def distance(self, p2=None):
    if p2 is None:
      p2 = Point.ORIGIN
    dx = self.x - p2.x
    dy = self.y - p2.y
    return (dx ** 2 + dy ** 2) ** .5

# attach ORIGIN after the Point class is defined
Point.ORIGIN = Point()

# we can access ORIGIN through the Point class.
print(Point.ORIGIN)
(0,0)

p1 = Point(3,4)
p2 = Point(3,19)

# Distance defaults to calculating how far away a Point is from ORIGIN
p1.distance()
5.0

# Distance will calculate the distance from one point to another if a
# a second Point is provided as a parameter.
p1.distance(p2)
15.0

Final Thoughts

Classes are an excellent example of how the Python programming language has benefitted from thorough design and community development. JavaScript was first created in 10 days by one person. Creating classes in JavaScript is a total pain. JavaScript is only recently gaining full-fledged, easy-to-use Object Oriented programming features that Python has had for a long time.