Class

Defining a class creates a new datatype. An object is an instance of a class (everything in python).

Attributes

An object whose datatype is of our class is called an instance of a class. For example 3 is an instance of the class int.

Each instance can hold more than just a singe piece of data, each of these pieces is called an instance attribute. You can think of this as nouns that describe the object.

Example of a Tweet as a class.

from datetime import date

class Tweet: 
	""""A tweet, like in Twitter.
	=== Attributes === 
	userid: the id of the user who wrote the tweet.
	created_at: the date the tweet was written. 
	content: the contents of the tweet.
	likes: the number of likes this tweet has received.
"""

# Attribute types, do not create instance variable and could be removed but is good practice and understood by automated tools
# client code
	userid: str 
	created_at: date
	content: str
	likes: int

	def __init__(self, who: str, when: date, what: str) -> None:
		self.userid = who
		self.created_at = when 
		self.content = what 
		self.likes = 0
	

*putting a _ in front of an attribute makes it "private"

Creating an instance: tweet = Tweet()
But in order to create and initialize instance attributes for a class we need to use the method __init__

self automatically receives the id of the instance that is to be initialized.

>>> t1 = Tweet('Giovanna', date(2022, 1, 18), 'Hello')


and after the return Tweet.__init__ we have:

we can access the attributes by using dot notation like t1.created_at

Methods

Functions inside the class, like the verbs related to the object. The benefit is that unlike a function, it doesn't require imports like a regular function; it is already associated with the class.

 class Tweet:
    ...

    def like(self, n: int) -> None:
        """Record the fact that <self> received <n> likes.

        Precondition: n >= 0
        """
        self.likes += n

tweet.like(10)

dot notation already passes self.

Can be called two ways, for example for strings

word = 'supercalifragilisticexpealidocious'
word.count('i')
# or 
str.count(word, 'i')

Special Methods

Representation Invariants

Making sure that rules are consistent and documented. A representation invariant is a property of the instance attributes that every instance of a class must satisfy. There are documented in the doctring.

Documentation:

class Tweet:
    """A tweet, like in Twitter.

    === Attributes ===
    userid: the id of the user who wrote the tweet.
    created_at: the date the tweet was written.
    content: the contents of the tweet.
    likes: the number of likes this tweet has received.

    === Representation Invariants ===
    - len(self.content) <= 280
    """
    # Attribute types
    userid: str
    created_at: date
    content: str
    likes: int

Enforcement
Assume that all representation invariants are satisfied but also make sure that at the end of the method it is still satisfied. So it is a precondition and a postcondition. However, you can temporarily violate it in the body of the code.

You can use initializer arguments to further enforce like:

def __init__(self, who: str, when: date, what: str) -> None:
    """Initialize a new Tweet.

    If <what> is longer than 280 chars, only first 280 chars are stored.
	Precondition: len(what)<= 280 # here is one option ************

    >>> t = Tweet('Rukhsana', date(2017, 9, 16), 'Hey!')
    >>> t.userid
    'Rukhsana'
    >>> t.created_at
    datetime.date(2017, 9, 16)
    >>> t.content
    'Hey!'
    >>> t.likes
    0
    """
    self.userid = who
    self.created_at = when
    self.content = what[:280] # or here ************
    self.likes = 0

However nothing fully prevents client code from violating representation invariants.

Designing Classes

Thus far there are 3 elements:

1. instance attributes (data)
2. methods (operations)
3. representation invariants (properties)

Design Recipe:
link

Main themes are, design before coding and information hiding (separation from the public interface).

Private-ness:

In python naming convention, anything considered private starts with a leading underscore. The client should not access or change the value. Python has a "we're all adults philosophy" so unlike other languages, editing the variable might not raise an error.

Class Composition

It's common for an instance of a class to have and attribute that refers to another class. For example, in Twitter, there may be a user class and a tweet class.