Miscellaneous Notes

__repr__ and __str__ methods

Two dunder (double underscore) methods that are very similar and often confused are repr and str. They both return a string representation of an object. In this example, they are functionally very similar, both returning a string about the person object. Both attributes name and age are included.

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __str__(self):
        return f"I'm {self.name}, and I'm {self.age} years old."

    def __repr__(self):
        return f"{type(self).__name__}(name='{self.name}', age={self.age})"

Though functionally similar, the difference is who they are intended for. repr is meant to be an unambiguous string representation of an object for developers to debug and/or recreate the object if necessary. str is used for a user friendly printed representation of an object for end-users. The focus here is on readability.

Just remember repr is for developers and str is for customers.

By default, printing an object will use the str method, unless there is none, in which case it will fall back to the repr method. if there is neither a repr nor a str method, it will print the memory address.

lamda

class Book:
    def __init__(self, title, author, pages):
        self.title = title
        self.author = author  
book_details = [
    ("It", "Stephen King", 1138),
    ("A Tale of Two Cities", "Charles Dickens", 304),
    ("Going Postal", "Terry Pratchett", 484)
]

books = []

for details in book_details:
    books.append(Book(*details))

sorted_books = sorted(books, key=lambda book: book.title)

In this example, there is a list of book objects called books created from information in a list of tuples book_details. An additional list is created called sorted_books that makes reference to the same objects in the previous list, but orders them based on the book title of each object. lambda is used as a concise, inline key for sorting. It provides a anonymous function without the need for a def keyword, such as the following.

def get_title(book):
    return book.title

sorted_books = sorted(books, key=get_title)