Design Elegant Functions: Closures, Decorators, and APIs

Design clear function signatures using positional-only, keyword-only, `*args`, and `**kwargs`.

Identify and correct the mutable default argument anti-pattern.

Pass functions as arguments and return them from other functions to implement callbacks and factories.

Write closures that capture state from their enclosing scope and use the `nonlocal` keyword to modify that state.

Use `functools.partial` for API simplification, `lru_cache` for memoization, and `singledispatch` for ad-hoc polymorphism.

Choose between `map`/`filter` and comprehensions/generators, using lambdas only for short, throwaway functions.

Design small, testable, pure functions with side-effects pushed to the edges of the application.