Unit 4 · Moderate
Classes in a language with no enforced privacy. Encapsulation here is a convention the programmer upholds rather than a rule the language imposes, which changes how it must be taught.
What a strong answer looks like
A strong Unit 4 answer distinguishes state that belongs to an instance from state that belongs to the class, and says which methods are allowed to change it.
Know
A class is a template; an instance is one object built from it. The initialiser runs at construction and sets up instance state.
Apply
Give every attribute a value in the initialiser so no instance starts half-built.
Watch out
Forgetting the instance parameter on a method, which produces a confusing argument-count error.
Study move
Write a class with two attributes and construct three instances with different values.
Know
An attribute set on the class is shared by every instance; one set in the initialiser belongs to that instance alone.
Apply
Put per-object data in the initialiser and shared constants on the class.
Watch out
A mutable class attribute being modified through one instance and appearing changed for all of them.
Study move
Create a class with a mutable class attribute, modify it via one instance, and observe the effect on another.
Know
Python marks intended privacy by naming convention rather than enforcement, so encapsulation is a contract between programmers.
Apply
Expose behavior through methods and treat underscore-prefixed names as internal.
Watch out
Assuming a leading underscore prevents access. It signals intent only.
Study move
Write a class whose internal representation could change without breaking callers who respect the convention.
Know
Defining the string representation and equality methods makes an object print usefully and compare sensibly.
Apply
Implement a readable representation for any class you will debug.
Watch out
Comparing two objects without defining equality, which compares identity and returns false for equal values.
Study move
Define equality on a small class and show two distinct instances comparing equal.
Connections and techniques that receive extra attention in this unit.
Related topics some schools attach to this unit and others leave out. Covered on request rather than assumed.
Modeling drill
Write a class representing a quiz result that prints readably and compares equal when the scores match, and say which attributes belong to the class rather than the instance.
