Discussion 8

Inheritance and String Representation

Antonio Kam
anto [at] berkeley [dot] edu

All slides can be found on
teaching.rouxl.es

Slides by Antonio Kam (anto@)

Announcements

  • HW 06 Released!
    • OOP/Inheritance
  • Ants Phase 1 due on Friday
    • Please finish it, the checkpoint is there to keep you on track and is super important
Slides by Antonio Kam (anto@)

Notes from last section

  • More of these diagrams; Honestly, no feedback other than that this discussion was great (😳). The diagrams were nice.
  • mini-lecture was helpful but a little fast today (but mostly bc OOP is confusing)
    • makes sense, oop is confusing, but we'll get more familiar with it as time goes on
  • If you had unlimited money to start a business, what would it be? (ie what type of business would you start)
    • i have no clue at all tbh, never thought of this
    • i literally sat here thinking for like 5 minutes and still couldn't come up with something
  • What are your favorite places to travel?
    • Hong Kong: food; tbh a lot of asian countries (food)
Slides by Antonio Kam (anto@)

Notes from last section

  • a little bit more on OOP
    • 🤯 this discussion is also oop related!
  • Mini-lecture was helpful + inheritance more clear (lab)
    • nice, we'll see a bit more
  • would you rather have unlimited bacon but no games. Or games, unlimited games, but no games.
    • ...what?
    • I assume the other clause says unlimited games but no bacon
    • and i think that choice is easy - unlimited games but no bacon
  • What is your fastest record for the rubik's cube? Could we see a demo 0.o
    • One Handed: 6.85 Single, 9.28 Average; Blindfolded: 17.47 Single
Slides by Antonio Kam (anto@)

Mid-Semester Feedback

  • Overall I think y'all like me (hopefully) :partying:
  • 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!
  • I speak too fast
    • many people said this, and it's true 😭 i speak very fast, and it's a reoccuring issue for me.
    • "Maybe try to talk a little bit slower, but I also understand that you kind of have to talk fast to get through everything" - this is not true, i can talk slow and do this

Mid-Semester Feedback

  • Move through the material slower, simplify the material
  • Sometimes we spend too much time on the first topic when there are multiple topics to be covered.
    • this is still a pretty bad problem for me, but i think i've been doing a better job of selecting the more important topics
    • still room to improve though
  • Many of you said I have a lot of energy 😳
    • This is something that I don't have normally (outside of teaching), but it's weird that this has consistently showed up for me teaching 🤔
  • "Very fun. A lot of energy to the room even when the classroom doesn't give much back."
    • hi can u give energy back pls 👉 🥺 👈

Mid-Semester Feedback

  • pacing, maybe more dispersed lecturing on content
    • i think in general this won't help too much with time management, just because the transitions tend to be what takes up more time, but i am still open to trying this
  • Mayhaps more coding examples
    • true
  • giving more guidance in lab / going over
    • we're all here during lab - it's a time for you to ask questions and even collaborate with the people near you!
  • switch on the lights at the front

Temperature Check 🌡️

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

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(Pet): # Cat inherits the Pet class - as in, all cats are pets (in this world)
    ...
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!

Slides by Antonio Kam (anto@)

Results from last section (links.rouxl.es/disc)

Slides by Antonio Kam (anto@)

Worksheet!

Slides by Antonio Kam (anto@)

Representation

Slides by Antonio Kam (anto@)

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@)

repr and str

  • In addition, the print() function calls the __str__ method of the object and displays the returned string with the quotations removed, while simply calling the object in interactive mode in the interpreter calls the __repr__ method and displays the returned string with the quotations removed.
  • show worksheet for examples anto
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!
Slides by Antonio Kam (anto@)

Worksheet!

Slides by Antonio Kam (anto@)

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Slides by Antonio Kam (anto@)

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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