Discussion 7: OOP, String Representation
OOP
Object-oriented programming (OOP) is a programming paradigm that allows us to treat data as objects, like we do in real life.
For example, consider the class Student
.
Each of you as individuals is an instance of this class.
Details that all CS 61A students have, such as name
,
are called instance variables.
Every student has these variables, but their values differ from student to student.
A variable that is shared among all instances of Student
is known as a class variable.
For example, the extension_days
attribute is a class variable
as it is a property of all students.
All students are able to do homework, attend lecture, and go to office hours.
When functions belong to a specific object, they are called methods.
In this case, these actions would be methods of Student
objects.
Here is a recap of what we discussed above:
- class: a template for creating objects
- instance: a single object created from a class
- instance variable: a data attribute of an object, specific to an instance
- class variable: a data attribute of an object, shared by all instances of a class
- method: a bound function that may be called on all instances of a class
Instance variables, class variables, and methods are all considered attributes of an object.
Q1: WWPD: Student OOP
Below we have defined the classes Professor
and Student
, implementing some of what was described above.
Remember that Python passes the self
argument implicitly to methods when calling the method directly on an object.
class Student:
extension_days = 3 # this is a class variable
def __init__(self, name, staff):
self.name = name # this is an instance variable
self.understanding = 0
staff.add_student(self)
print("Added", self.name)
def visit_office_hours(self, staff):
staff.assist(self)
print("Thanks, " + staff.name)
class Professor:
def __init__(self, name):
self.name = name
self.students = {}
def add_student(self, student):
self.students[student.name] = student
def assist(self, student):
student.understanding += 1
def grant_more_extension_days(self, student, days):
student.extension_days = days
What will the following lines output?
>>> callahan = Professor("Callahan")
>>> elle = Student("Elle", callahan)
>>> elle.visit_office_hours(callahan)
>>> elle.visit_office_hours(Professor("Paulette"))
>>> elle.understanding
>>> [name for name in callahan.students]
>>> x = Student("Vivian", Professor("Stromwell")).name
>>> x
>>> [name for name in callahan.students]
>>> elle.extension_days
>>> callahan.grant_more_extension_days(elle, 7)
>>> elle.extension_days
>>> Student.extension_days
Q2: Email
We would like to write three different classes (Server
, Client
,
and Email
) to simulate a system for sending and receiving email.
To solve this problem, we'll split the section into two halves (students on the left and students on the right):
- Everyone will implement the
Email
class together - The first half (left) will implement the
Server
class - The other half (right) will implement the
Client
class
Fill in the definitions below to finish the implementation!
Run in 61A CodeInheritance
To avoid redefining attributes and methods for similar classes, we can write a single base class from which the similar classes inherit. For example, we can write a class called Pet and define Dog as a subclass of Pet:
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):
def talk(self):
super().talk()
print('This Dog says woof!')
Inheritance represents a hierarchical relationship between two or more
classes where one class is a more specific version of the other:
a dog is a pet
(We use is a to describe this sort of relationship in OOP languages,
and not to refer to the Python is
operator).
Since Dog
inherits from Pet
, the Dog
class will also inherit the
Pet
class's methods, so we don't have to redefine __init__
or eat
.
We do want each Dog
to talk
in a Dog
-specific way,
so we can override the talk
method.
We can use super()
to refer to the superclass of self
,
and access any superclass methods as if we were an instance of the superclass.
For example, super().talk()
in the Dog
class will call the talk()
method from the Pet
class, but passing the Dog
instance as the self
.
This is a little bit of a simplification,
and if you're interested you can read more
in the
Python documentation
on super
.
Q3: Cat
Below is a skeleton for the Cat
class, which inherits from
the Pet
class. To complete the implementation, override the
__init__
and talk
methods and add a new
lose_life
method.
Run in 61A CodeHint: You can call the
__init__
method ofPet
(the superclass ofCat
) to set a cat'sname
andowner
.
Q4: NoisyCat
More cats! Fill in this implementation of a class called
NoisyCat
, which is just like a normal Cat
. However,
NoisyCat
talks a lot: in fact, it talks twice as much as a regular Cat
!
If you'd like to test your code, feel free to copy over your solution to the Cat
class above.
Class Methods
Now we'll try out another feature of Python classes: class methods.
A method can be turned into a class method by adding the
classmethod
decorator.
Then, instead of receiving the instance as the first argument (self
),
the method will receive the class itself (cls
).
Class methods are commonly used to create "factory methods": methods whose job is to construct and return a new instance of the class.
For example, we can add a robo_factory
class method to our Dog
class
that makes robo-dogs:
class Dog(Pet):
# With the previously defined methods not written out
@classmethod
def robo_factory(cls, owner):
return cls("RoboDog", owner)
Then a call to Dog.robo_factory('Sally')
would return a new Dog
instance
with the name "RoboDog" and owner "Sally".
Q5: Own A Cat
Now implement the cat_creator
method below,
which takes in a string owner
and creates a Cat
named "[owner]'s Cat", where [owner] is replaced with the name in the owner
string.
Hint: To place an apostrophe within a string, the entire string must be surrounded in double-quotes (i.e. "DeNero's Dog"
)
Representation: Repr, Str
There are two main ways to produce the "string" of an object in Python:
str()
and repr()
. While the two are similar, they are
used for different purposes.
str()
is used to describe
the object to the end user in a "Human-readable" form,
while repr()
can be thought of as a "Computer-readable"
form mainly used for debugging and development.
When we define a class in Python, __str__
and __repr__
are
both built-in methods for the class.
We can call those methods using the
global built-in functions str(obj)
or repr(obj)
instead of
dot notation,
obj.__repr__()
or obj.__str__()
.
In addition, the print()
function calls the __str__
method of the object,
while simply calling the object in interactive mode calls the _repr__
method.
Here's an example:
class Rational:
def __init__(self, numerator, denominator):
self.numerator = numerator
self.denominator = denominator
def __str__(self):
return f'{self.numerator}/{self.denominator}'
def __repr__(self):
return f'Rational({self.numerator},{self.denominator})'
>>> a = Rational(1, 2)
>>> str(a)
'1/2'
>>> repr(a)
'Rational(1,2)'
>>> print(a)
1/2
>>> a
Rational(1,2)
Q6: WWPD: Repr-esentation
Note: This is not the typical way
repr
is used, nor is this way of writingrepr
recommended, this problem is mainly just to make sure you understand howrepr
andstr
work.
class A:
def __init__(self, x):
self.x = x
def __repr__(self):
return self.x
def __str__(self):
return self.x * 2
class B:
def __init__(self):
print('boo!')
self.a = []
def add_a(self, a):
self.a.append(a)
def __repr__(self):
print(len(self.a))
ret = ''
for a in self.a:
ret += str(a)
return ret
Given the above class definitions, what will the following lines output?
>>> A('one')
>>> print(A('one'))
>>> repr(A('two'))
>>> b = B()
>>> b.add_a(A('a'))
>>> b.add_a(A('b'))
>>> b