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cs_lec_13

#r/lecture

  • Lecture notes: link

Lecture: 10/10

print("Hello world i am cool")

Attributes

Terminology: Attributes, Functions and Methods

  • All objects have attributes, which are name-value pairs
  • Classes are objects too, so they have attributes
  • Instance attribute: attribute of an instance
  • Class attribute: attribute of a class
    • probably equivalent to methods and static objects in Java.

Terminology: ![[chrome_PBI7Kk8h3w.png]]

Python Object System

  • Functions are objects.
  • Bound methods are also objects: a function that has its first parameter "self" already bound to an instance (of the class)
  • Dot expressions evalate to bound methods for class attributes that are functions. 
    - Example: 
        - instance.method_name
    Looking up attributes by name(reminder) expression . name To evaluate dot expression
  1. Evaluate expression to the left of the dot, which yields the object(required)
  2. Name is matched against the instance attributes of that object; if an attribute with that name exists, its vlaue is returned.
  3. If not, name is looked up in the class, which yields a class attribute value
  4. That value is returned unless its a function, in which case a bound method is returned instaed

Assignment statements with a dot expression on their left-hand side affect attirbutes for the objet of that dot expression.

  • If the object is instance, then assignemnt set sinstance attribute
  • If object is class, then assignment sets a class attribute. Instance attribute assignment example: ![[chrome_xR4KuTK9I4.png]] 'interest' is an instance attribute, since the object of the dot expression(the first thing before the dot) is tom_account, which is an instance (of class Account)

The name (interest) is not looked up in the class. In fact, tom_account didn't even have an attribute interest, only holder and balance, since it wasn't defined in the constructor.

So basically python creates a new attribute called interest under tom_account.

Class attribute Assignment ![[chrome_T9WDh3sCM3.png]] Account exists and is a class, so this is class attirbute assignment. Since the interest attribute already exists, that attribute under Account will be modified.

Attribute Assignment Statements For class assignemnt ![[chrome_eh7xEeJrw3.png]] So interest is the attribute for every class. (withdraw, deposit) are methods. ![[chrome_S3ePVAH28r.png]] As seen above, we see that the interest attribute is not static and can be changed, and thsi change will be reflected in all of the instances of the class Account For instance assignemnt ![[chrome_POhwzuOr54.png]]

  • Instance assignment attributes in different objects are different and can take on different values, even if both instances involved are objects of the same class
  • The instance assignment (name) can be the same as a class attribute, and that will be completely fine. The assignment of the attribute for the instance is independent from the class attribute.

Inheritance Inheritance -> Relating classes together Common use: Two similar classes differ in degree of specialization. 1) Specialized class may have same attributes as general class, along with special-case behaviour 2) New subclass "shares" attr with base class 3) Subclass may override inherited attributes, class or instance.

Example Inheritance Checking account -> specialized Account (with interest cuz banks are trash) ![[chrome_rPc9uMpm0X.png]] 1) Basically regular account, but withdraw method is stlightly different. 2) Perfect solution is to have the Checking account extend/inherit from Account, but overriding the withdraw method to add the fee which is why banks are trash Extending in python example below: ![[chrome_d9njr1ZSxy.png]] Base class attributes aren't copied into subclasses! To look up name in class:

  1. If it names attribute in class, return attribute
  2. Otherwise, look up name in base class, if there is one. ![[chrome_ZHcNRwryPE.png]] Noob implementation(w.o extend of base class method): ![[chrome_32M7XkEWus.png]] Dominant implementation ![[chrome_URDxhHACbk.png]]
  • Using parent method to accomplish goals ![[chrome_dsiFyxJF8Z.png]] ![[chrome_maVQTUafYY.png]]

Inheritance & Composition ![[chrome_5BwlHsh1MR.png]]

![[chrome_rItrPfyV0y.png]]

Designing for Inheritance ![[chrome_Gzh4lLrqDC.png]] ![[chrome_Cjd2iC5jD0.png]]

WTF: ![[chrome_1KOYMYwIKm.png]]

Multiple Inheritance ![[chrome_32s8dM7gHq.png]] ![[chrome_1yvg4wQuEl.png]]

![[chrome_JPeGZE4j1g.png]] Only thing we need to know is that methods will first be pulled from the (first level parents class), rather than the upper levels parent class.

There is a particular order in which you look at all of this. To find (attributes, methods, etc), all one needs to know is that you look in the subclasses before you look at the base class - John DeNero.