OOP: basic-level questions

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Variables: Conceptual Questions

Question 1

Define each of the following terms:

  1. Local variable
  2. Instance variable
  3. Class variable
  1. Local variable: a variable that is only visible within the scope of a method. Once the method finishes executing, the local variable is erased.

  2. Instance variable: a variable that persists — even after methods are done executing, these variables will still exist and retain their value.

    • Tip: you can tell a variable is an instance variable if it has self. in front of it (e.g. self.name).
    • Instance variables can only be used within methods.
    • Instance variables are unique to each instance of the class. They are not shared by instances.

  3. Class variable: like instance variables, class variables also persist. However, class variables ARE shared by all instances of the class.

    • When initialized outside of methods (which is usually the case), the class variable has no "dot" modifier (e.g. just num_of_accounts
    • When referenced in methods, the class variable must be referenced with the following syntax: class_name.variable (e.g. Account.num_of_accounts)

Question 2

Consider the following code:

class Account:
    interest = 0.02
    def __init__(self, name, balance):
        self.name = name
        self.balance = balance

    def deposit(self, amt):
        total = self.balance + amt
        self.balance = total

Determine whether each of these variables are local, instance, or class variables:

  1. name
  2. self.name
  3. balance
  4. self.balance
  5. interest
  6. amt
  7. total
  1. name: local
  2. self.name: instance
  3. balance: local
  4. self.balance: instance
  5. interest: class
  6. amt: local
  7. total: local

Question 3

Consider the following code:

class Person:
    def __init__(self, name):
        self.name = name

Let's say we want to have a variable that keeps track of all the Person objects ever created.

  • What type of variable should this be? (local, instance, or class)
  • Modify the code to initialize population to 0, and to increment it by 1 every time you create a new Person object.

This would be a class variable. The new code would look like this:

class Person:
    population = 0
    def __init__(self, name):
        self.name = name
        Person.population += 1

Variables: What would Python print?

For the following questions, use the following class definition:

class Account:
    """A class computer account. Each account has a two-letter ID
    and the name of the student who is registered to the account.
    """
    num_of_accounts = 0
    def __init__(self, id):
        self.id = id
        Account.num_of_accounts += 1

    def register(self, student):
        self.student = student
        print('Registered!')

    @property
    def type(self):
        return type(self)

Question 4

>>> self.id
______
>>> acc_aa = Account("aa")
>>> acc_aa.id
______
>>> acc_aa.student
______
>>> acc_aa.register("Peter Perfect")
______
>>> acc_aa.student
______
>>> num_of_accounts
______
>>> Account.num_of_accounts
______
>>> acc_aa.num_of_accounts
______
>>> acc_zz = Account("zz")
>>> Account.num_of_accounts
______
>>> acc_aa.num_of_accounts
______
>>> acc_zz.num_of_accounts
______
>>> acc_aa.num_of_accounts = 100
>>> acc_aa.num_of_accounts
______
>>> acc_zz.num_of_accounts
______
>>> Account.num_of_accounts
______
>>> Account.num_of_accounts = 9001
>>> acc_aa.num_of_accounts
______
>>> acc_zz.num_of_accounts
______

Methods: Conceptual Questions

Question 5

Here is the same Account class from the previous section:

class Account:
    """A class computer account. Each account has a two-letter ID
    and the name of the student who is registered to the account.
    """
    num_of_accounts = 0
    def __init__(self, id):
        self.id = id
        Account.num_of_accounts += 1

    def register(self, student):
        self.student = student
        print('Registered!')

    @property
    def type(self):
        return type(self)

Why is it that, when I call acc_aa.register('me') no errors will be raised, even though I didn't pass in an argument for self?

The dot notation will implicitly pass acc_aa into type as self. This is known as a bound method. Another way to think about it is that acc_aa.register acts like a curried function

>>> acc_aa.register = curry2(Account.register)(acc_aa)
>>> acc_aa.register('me')
Registered!

Question 6

Can a method have the same name as a variable?

No; in python, variables and methods share the same namespace, so variable and method names can collide if you aren't careful.

Question 7

What does the @property decorator do?

The @property decorator allows you to use the affected method to be accessed like a variable. For example, the following method

>>> class Example:
...     @property
...     def foo(self):
...         return 3""") + """can be accessed like
...         this:""" + prettify("""
>>> a = Example()
>>> a.foo
3

Methods: What would Python print?

For the following questions, use the Account class defined below:

class Account:
    """A class computer account. Each account has a two-letter ID
    and the name of the student who is registered to the account.
    """
    num_of_accounts = 0
    def __init__(self, id):
        self.id = id
        Account.num_of_accounts += 1

    def register(self, student):
        self.student = student
        print('Registered!')

    @property
    def type(self):
        return type(self)

Question 8

>>> acc_aa = Account("aa")
>>> acc_aa.register
______
>>> Account.register
______
>>> acc_aa.register(self, "Peter Perfect")
______
>>> acc_aa.register("Peter Perfect")
______
>>> acc_aa.type()
______
>>> acc_aa.type
______
>>> acc_aa.type = "Nothing"
______