chapter_10_design_patterns_fp

Chapter 10 — Design Patterns with First-Class Functions

Theme: Many classic Gang of Four (GoF) design patterns are unnecessarily complex in Python. Because functions are first-class objects, we can replace heavyweight Abstract Base Classes with simple Callable references.

What You'll Learn

Topic	Key insight
Peter Norvig's Insight	In 1996, Norvig proved that 16 of the 23 GoF patterns are invisible or simpler in languages with first-class functions.
The Strategy Pattern	Replace abstract Strategy classes with simple functions.
The Command Pattern	Replace Invoker interfaces with callables or closures.
The Template Method	Pass functions to algorithms instead of forcing class inheritance.
The "Java-in-Python" Anti-pattern	Writing classes with exactly one method (besides __init__) is usually a mistake in Python.

Key Files

• examples.py — Side-by-side refactoring of OOP Strategy/Command to FP.
• exercises.py — Refactoring the Template Method and building a Command Queue.
• mini_project.py — E-commerce checkout applying Strategy (promotions) and Command (event hooks).
• benchmarks.py — Proves that passing function references is ~2x faster than instantiating Strategy objects.
• notes.md — Theory on pattern simplification.
• pitfalls.md — When not to use functions (i.e., when state gets too complex).
• interview_questions.md — L3 to L6 interview prep.
• architecture_notes.md — CPython's bounded methods as implicit Command objects.

30-Second Rules

# Rule 1: Replace single-method classes with functions
# BAD:
class Discount(ABC):
    @abstractmethod
    def apply(self, price): pass
class VIPDiscount(Discount):
    def apply(self, price): return price * 0.8

# GOOD:
def vip_discount(price): return price * 0.8

# Rule 2: Use lists of Callables for MacroCommands
hooks = [send_email, update_db, clear_cache]
for hook in hooks: hook()

Reference: Fluent Python 2nd ed., Chapter 10 — pages 381–402