Discussion 7

OOP and String Representation

Antonio Kam
anto [at] berkeley [dot] edu

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Slides by Antonio Kam (anto@)

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Slides by Antonio Kam (anto@)

Results from last section

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Notes from last section

  • OOP/Objects: when would you use OOP over another programming paradigm
    • Good question! OOP is very useful if there's a hierarchical structure to what you want to do (for example, video games tend to have this structure with enemies; you probably saw this with Ants, where implementing new ants took far less effort than implementing the 'first' ant)
  • More on super and inheritance
    • This discussion!
  • "Maybe ask if there are people working on a specific question midway through lab so that people can work together"
    • Good idea - I'll try something like this next lab
  • I really enjoy the mini lectures!
    • 😳
Slides by Antonio Kam (anto@)

Notes from last section

  • I SUMMON POT OF GREED, THIS ALLOWS ME TO DRAW 3 CARDS
    • 🥳 thanks for this comment
    • Also would highly recommend doing the optional questions on the last lab - they're pretty useful for understanding more about how OOP works
  • Recursion Practice
    • You'll get... quite a lot on this soon 👀
    • The rest of the course from here is pretty much recursion until the end!
Slides by Antonio Kam (anto@)

Mid-Semester Feedback

  • Sometimes there's very contrasting feedback
    • Some people want discussions to go faster
    • Some people want discussions to go slower
    • I unfortunately cannot accommodate for all of this
    • Discussion review sections!
    • Also, worksheets are designed to be longer than what can be covered
  • I speak too fast
    • I knew this was an issue 😭 - I do tend to speak fast when I get excited, which is not a great habit
    • Feel free to scream at me if I'm speaking too fast - you can also ask me to go over something again, and I'm more than happy to do that for you
Slides by Antonio Kam (anto@)

Mid-Semester Feedback

  • Lab mini-lectures are too long
    • You're more than welcome to start doing the lab even while I'm talking
    • Even asking questions is fine: one of the AIs will get to you if it's a lab based question
  • More group discussions/engagement
    • Please help me with this 🥺 - would like this to improve too!
    • I might try 'enforcing' something next discussion if there's not enough group discussion here
    • I promise group discussion is useful (Lost on the Moon!)
  • Many of you said I have a lot of energy 😳
    • This is something that I don't have ever, unless I'm teaching, which I find super strange 🤔
Slides by Antonio Kam (anto@)

Temperature Check 🌡️

  • OOP
  • Inheritance
  • Class Methods
  • Representation (repr, str)
Slides by Antonio Kam (anto@)

Object-Oriented
Programming

Slides by Antonio Kam (anto@)

What is OOP?

  • One way I like to think of OOP is as a sort of 'advanced' data abstraction
    • You would use OOP for similar things that you would use data abstractions for
    • Can make a City class (similar to the City data abstraction you messed around with during Lab)
  • OOP also allows for inheritance (less repetition of code, more on this later)
  • OOP also allows for mutation
    • Similar to list mutation (.append, .extend, etc.)
  • You may have seen this if you've seen Java before (I didn't have any exposure to OOP when I took CS 61A for the first time)
Slides by Antonio Kam (anto@)

OOP Terminology

  • Class
    • A class is sort of a 'blueprint' for something. You can think of it as a template for creating an object
  • Instance
    • An instance of a class is one object of that blueprint, or one physical object that you create based on your template
  • Variables
    • Instance Variables: Variables unique to each instance
    • Class Variables: Variables shared between each instance in the same class
  • Method
    • Function bound to a class
Slides by Antonio Kam (anto@)

Functions vs Methods

  • Methods need to take in self as an argument
    • This is very often implicitly passed in when the thing on the left side of the dot is an instance
    • my_car.drive(100) is the same as Car.drive(my_car, 100)
  • self refers to the actual instance (rather than the class)
  • Two ways of calling methods:
    • Class.method(self, args)
    • instance.method(args)
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Q1 Mini-lecture

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Worksheet!

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Inheritance

class Dog():
    def __init__(self, name, owner):
        self.is_alive = True
        self.name = name
        self.owner = owner
    def eat(self, thing):
        print(self.name + " ate a " + str(thing) + "!")
    def talk(self):
        print(self.name + " says woof!")

class Cat():
    def __init__(self, name, owner, lives=9):
        self.is_alive = True
        self.name = name
        self.owner = owner
        self.lives = lives
    def eat(self, thing):
        print(self.name + " ate a " + str(thing) + "!")
    def talk(self):
        print(self.name + " says meow!")
Slides by Antonio Kam (anto@)

Inheritance

Notice the redundancies in the code? One of the core foundations in this class is to not repeat yourself (DRY)

  • Instead, you can use inheritance to solve this problem
  • Syntax when creating a class is to put brackets around the class you want to inherit:
class Cat(Animal): # Cat inherits the Animal class - as in, all cats are animals
    ...
Slides by Antonio Kam (anto@)

Inheritance

class Pet():
    def __init__(self, name, owner):
        self.is_alive = True    # It's alive!!!
        self.name = name
        self.owner = owner
    def eat(self, thing):
        print(self.name + " ate a " + str(thing) + "!")
    def talk(self):
        print(self.name)

class Dog(Pet): # Inherits all methods/variables from the Animal class
    def talk(self):
        print(self.name + ' says woof!')
Slides by Antonio Kam (anto@)

Inheritance - super()

  • Calling super() will refer to the class's superclass
  • You can use the parent's method and then add on to that.
class Cat(Pet): # Inherits all methods/variables from the Animal class
    def __init__(self, name, owner, lives = 9):
        super().__init__(name, owner)
        # same as calling Pet.__init__(self, name, owner) from here
        self.lives = 9
    def talk(self):
        print(self.name + ' says meow!')
Slides by Antonio Kam (anto@)

Worksheet!

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Class Methods

class Dog(Pet):
    # With the previously defined methods not written out
    @classmethod
    def robo_factory(cls, owner):
        return cls("RoboDog", owner)
  • Uses the @classmethod decorator
  • Useful for when you want to create multiple instances of a class based on information you already have
  • Also useful when you have a method for a class that may not necessarily be associated with an instance (e.g. Path.cwd())
  • For example, in the case above, if I want to create a bunch of dogs with a certain owner, I can use this Class Method
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Attendance
links.rouxl.es/disc

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Worksheet!

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Representation

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What is Representation?

  • Python objects by default have really ugly names if you try to output them into the interpreter:
    • <__main__.Cat object at 0x7fe611abff70>
  • Representation lets you define a better way to display this out in the console such that other people know what you're talking about
Slides by Antonio Kam (anto@)

repr and str

  • repr
    • Affects what is displayed when object is evaluated in terminal
    • 'Computer readable' (as in, you can use eval on something from a __repr__ method, and it should not error if implemented correctly)
  • str
    • Changes what is displayed when an object is printed
    • 'Human readable'
  • If you directly output to a console, __repr__ is used, but if you try to print, it will first try and find __str__, and if that doesn't exist, it will use __repr__ instead.
Slides by Antonio Kam (anto@)

Example

class Pet:
    def __init__(self, name):
        self.name = name
    
    def __repr__(self):
        return "Good " + self.name

    def __str__(self):
        return self.name + " says hello!"
  • rex = Pet("Rex")
  • rex -> Good Rex (notice how this doesn't have quotes? Python behaviour!)
  • print(rex) -> Rex says hello!
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Worksheet!

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Thanks for coming! 🥳

Please give me feedback on what to improve!

Slides by Antonio Kam (anto@)

Draw environment diagram thing for this