Planning a Class Structure

Planning a Class Structure#

Note

Source: Adapted from the C# edition (classes/planning_classes.rst). The design principles — identify nouns as classes, verbs as methods, data as instance variables — are language-agnostic.

Before writing code, it pays to think about what classes you need and what each class is responsible for.

Nouns and Verbs#

A reliable starting point: read the problem description and

  • Nouns (especially those that appear in more than one place, or that have several pieces of data attached) suggest classes.

  • Verbs associated with a noun suggest methods of that class.

  • Adjectives or data attributes suggest instance variables.

Consider a simple library system. The problem mentions books, patrons, and loans. A book has a title, author, and ISBN. A patron has a name and a library card number. A loan connects a book to a patron and records a due date. From this, three classes emerge naturally: Book, Patron, and Loan.

Identifying Instance Variables and Methods#

For each class, ask:

  1. What data defines an instance? These become instance variables.

  2. What can an instance do, or have done to it? These become methods.

  3. How should an instance be displayed as a string? That’s __str__.

For a Book class:

  • Data: title, author, isbn

  • Methods: __init__, __str__, is_available()

Keep each class focused on one responsibility. If a class is doing too many unrelated things, split it.

Relationships Between Classes#

Classes interact through their public methods. A Loan holds references to a Book and a Patron:

class Loan:
    def __init__(self, book, patron, due_date):
        self.book = book
        self.patron = patron
        self.due_date = due_date

    def __str__(self):
        return f"{self.patron} borrowed {self.book} (due {self.due_date})"

This pattern — one class holding a reference to another — is called composition. It is the main way to build larger systems from smaller classes.

A Checklist#

Before writing a class, be able to answer:

  • What instance variables does it have, and what type is each?

  • What does __init__ receive as arguments?

  • What methods does it expose to the rest of the program?

  • What should __str__ return?

If you can answer these questions clearly, writing the code is usually straightforward.