WEEK 5 :: PYTHON CRASH COURSE :: OBJECT ORIENTED PROGRAMMING

Creating new instances of class objects can be a great way to keep track of values using attributes associated with the object. The values of these attributes can be easily changed at the object level. The following code illustrates a famous quote by George Bernard Shaw, using objects to represent people. Fill in the blanks to make the code satisfy the behavior described in the quote.

# "If you have an apple and I have an apple and we exchange these apples then # you and I will still each have one apple. But if you have an idea and I have # an idea and we exchange these ideas, then each of us will have two ideas." # George Bernard Shaw class Person: apples = 0 ideas = 0 johanna = Person() johanna.apples = 1 johanna.ideas = 1 martin = Person() martin.apples = 2 martin.ideas = 1 def exchange_apples(you, me): #Here, despite G.B. Shaw's quote, our characters have started with #different amounts of apples so we can better observe the results. #We're going to have Martin and Johanna exchange ALL their apples with #one another. #Hint: how would you switch values of variables, #so that "you" and "me" will exchange ALL their apples with one another? #Do you need a temporary variable to store one of the values? #You may need more than one line of code to do that, which is OK. x=0 x=you.apples you.apples=me.apples me.apples=x return you.apples, me.apples def exchange_ideas(you, me): #"you" and "me" will share our ideas with one another. #What operations need to be performed, so that each object receives #the shared number of ideas? #Hint: how would you assign the total number of ideas to #each idea attribute? Do you need a temporary variable to store #the sum of ideas, or can you find another way? #Use as many lines of code as you need here. x=0 x=you.ideas you.ideas += me.ideas me.ideas += x return you.ideas, me.ideas exchange_apples(johanna, martin) print("Johanna has {} apples and Martin has {} apples".format(johanna.apples, martin.apples)) exchange_ideas(johanna, martin) print("Johanna has {} ideas and Martin has {} ideas".format(johanna.ideas, martin.ideas))

The City class has the following attributes: name, country (where the city is located), elevation (measured in meters), and population (approximate, according to recent statistics). Fill in the blanks of the max_elevation_city function to return the name of the city and its country (separated by a comma), when comparing the 3 defined instances for a specified minimal population. For example, calling the function for a minimum population of 1 million: max_elevation_city(1000000) should return "Sofia, Bulgaria".

# define a basic city class class City: name = "" country = "" elevation = 0 population = 0 # create a new instance of the City class and # define each attribute city1 = City() city1.name = "Cusco" city1.country = "Peru" city1.elevation = 3399 city1.population = 358052 # create a new instance of the City class and # define each attribute city2 = City() city2.name = "Sofia" city2.country = "Bulgaria" city2.elevation = 2290 city2.population = 1241675 # create a new instance of the City class and # define each attribute city3 = City() city3.name = "Seoul" city3.country = "South Korea" city3.elevation = 38 city3.population = 9733509 def max_elevation_city(min_population): # Initialize the variable that will hold # the information of the city with # the highest elevation return_city = City() # Evaluate the 1st instance to meet the requirements: # does city #1 have at least min_population and # is its elevation the highest evaluated so far? if city1.population >= min_population and city1.elevation > return_city.elevation: return_city = city1 # Evaluate the 2nd instance to meet the requirements: # does city #2 have at least min_population and # is its elevation the highest evaluated so far? if city2.population >= min_population and city2.elevation >return_city.elevation: return_city = city2 # Evaluate the 3rd instance to meet the requirements: # does city #3 have at least min_population and # is its elevation the highest evaluated so far? if city3.population >= min_population and city3.elevation > return_city.elevation: return_city = city3 #Format the return string if return_city.name: return "{}, {}".format(return_city.name, return_city.country) else: return "" print(max_elevation_city(100000)) # Should print "Cusco, Peru" print(max_elevation_city(1000000)) # Should print "Sofia, Bulgaria" print(max_elevation_city(10000000)) # Should print ""

You want to find more information about the integer (int) class. What's the best way to do this?

Use the command help(int) (Using the help command can be useful for finding quick documentation about the methods in a class.)

Let's test your knowledge of using dot notation to access methods and attributes in an object. Let's say we have a class called Birds. Birds has two attributes: color and number. Birds also has a method called count() that counts the number of birds (adds a value to number). Which of the following lines of code will correctly print the number of birds? Keep in mind, the number of birds is 0 until they are counted!

bluejay.count() print(bluejay.number) (We must first call the count() method, which will populate the number attribute, allowing us to print number and receive a correct response.)

class ___: color = 'unknown' rose = Flower() rose.color = ___ violet = ___ ___ this_pun_is_for_you = ___ print("Roses are {},".format(rose.color)) print("violets are {},".format(violet.color)) print(this_pun_is_for_you)

class Flower: color = "unknown" rose = Flower() rose.color = "red" violet = Flower() violet.color = "blue" this_pun_is_for_you = Flower() print("Roses are {},".format(rose.color)) print("violets are {},".format(violet.color)) print(this_pun_is_for_you)

We have two pieces of furniture: a brown wood table and a red leather couch. Fill in the blanks following the creation of each Furniture class instance, so that the describe_furniture function can format a sentence that describes these pieces as follows: "This piece of furniture is made of {color} {material}"

class Furniture: color = "" material = "" table = Furniture() table.color = "brown" table.material = "wood" couch = Furniture() couch.color = "red" couch.material = "leather" def describe_furniture(piece): return ("This piece of furniture is made of {} {}".format(piece.color, piece.material)) print(describe_furniture(table)) # Should be "This piece of furniture is made of brown wood" print(describe_furniture(couch)) # Should be "This piece of furniture is made of red leather"

What makes an object different from a class?

An object is a specific instance of a class (Objects are an encapsulation of variables and functions into a single entity.)

What Is a Method?

Calling methods on objects executes functions that operate on attributes of a specific instance of the class. This means that calling a method on a list, for example, only modifies that instance of a list, and not all lists globally. We can define methods within a class by creating functions inside the class definition. These instance methods can take a parameter called self which represents the instance the method is being executed on. This will allow you to access attributes of the instance using dot notation, like self.name, which will access the name attribute of that specific instance of the class object. When you have variables that contain different values for different instances, these are called instance variables.

Attributes are characteristics associated with a type. Methods are:

Functions associated with a type. (Remember, a method defines what you do with an object.)