What Is a Design Pattern
A design pattern is a named, proven solution to a problem that keeps coming back in software design. Patterns are not code you copy — they are shapes you recognize: "this is a wrapping problem", "this is a notification problem".
The classic catalog contains 23 patterns, collected by four authors known as the Gang of Four. Knowing them gives you two things: ready answers to common problems, and a shared vocabulary — saying "use an adapter here" replaces a whole paragraph of explanation.
The Three Families
The 23 patterns fall into three families:
Creational — how objects get made: Singleton, Factory Method, Abstract Factory, Builder, Prototype.
Structural — how things fit together: Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy.
Behavioral — how things talk to each other: Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template Method, Visitor.
Each pattern in this group has its own page: what it is, one runnable example, when to use it, and its pros and cons.
Patterns the Python Way
The catalog was written for languages built around classes and inheritance. Python has both, so the patterns translate directly — but it also has duck typing and first-class functions, and every page here uses them where they help.
Some patterns become much simpler in Python (a strategy is just a function, an iterator is just a generator); a few are barely needed at all. Each page says so honestly.
One warning before you start: patterns are tools, not goals. Using a pattern where a plain function would do makes code worse. Reach for a pattern when you feel its problem — never before.