Recap: What Design Patterns Are

Four elements, three families, the principle underneath, and the cost of using one unasked.

A design pattern is a named solution to a recurring design problem, a shape of classes to write in your own language rather than code to install.

Four elements. The Gang of Four describe each of their 23 patterns by its name, the problem it addresses, the solution's classes and responsibilities, and its consequences.
Recurring means recognisable. The iterator behind a Python for loop, a Java for-each and a TypeScript for...of is one pattern with the same two roles in three syntaxes.
Three families. Creational patterns decide how objects are made, structural patterns how they fit together, behavioural patterns how they divide work. An adapter is structural even though it changes what a call does.
Principles underneath. Most patterns apply a SOLID principle to a common problem: Strategy is open/closed, Adapter and Observer are dependency inversion, Composite relies on Liskov substitution.
Python absorbs some. First-class functions turn a one-method Strategy or Command into a function, generators replace a hand-written Iterator, and a module can be a Facade. The pattern's roles are still there, without the ceremony.
Every pattern costs something. Use one when the change it protects against has happened or is expected, and write the plain version until then.

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