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Chapter 10: Design Patterns with First-Class Functions — Notes

1. The Core Premise

In 1996, Peter Norvig noted that 16 out of the 23 design patterns in the original Design Patterns book (Gamma et al.) are "invisible or simpler" in languages with first-class functions (like Lisp, or Python). Python's ability to treat functions as objects makes many heavyweight OOP patterns redundant.

2. The Strategy Pattern

The Goal: Define a family of algorithms, encapsulate each one, and make them interchangeable.

Classic OOP implementation:

  1. Context object Order holds a reference to a Strategy interface.
  2. Abstract Base Class PromotionStrategy defines a .discount() method.
  3. Concrete subclasses VIPPromotion, BulkPromotion implement .discount().

Pythonic implementation:

  1. A strategy is just a Callable passed as a parameter.
  2. No Abstract Base Classes. No single-method subclasses.
  3. You just pass vip_promo_function directly to the Order.

Benefits: Less boilerplate. Functions are lighter than class instances.

3. The Command Pattern

The Goal: Decouple the object that invokes an operation from the one that knows how to perform it.

Classic OOP implementation: An Invoker holds an instance of an AbstractCommand and calls its .execute() method.

Pythonic implementation: An Invoker holds a Callable and calls func(). If the command requires state (arguments to be executed later), you can use functools.partial or a closure to freeze the arguments, entirely bypassing the need for a custom class with an __init__ and execute() method.

4. The MacroCommand

If you need to execute a sequence of commands, you don't need a complex Composite pattern. A MacroCommand in Python can simply be a class that stores a list of Callable objects and implements __call__ to iterate through them.

5. Identifying the "Java-in-Python" Anti-pattern

If you find yourself writing a class with:

  1. An __init__ method.
  2. Exactly one other method (like execute, run, or apply).

...you are likely writing Java in Python. That class should almost certainly be a simple function or a closure.