Closures & nonlocal

Why this matters

Every decorator, every functools.partial, and every FastAPI dependency-injection callable is a closure under the hood. Understanding late binding versus early binding is why senior engineers write def f(x, _cached=expensive()) and why lambda i: i inside a loop is a classic bug in JavaScript-to-Python converts.

Common pitfalls

• Late-binding in loops — [lambda: i for i in range(3)] all return 2; use lambda i=i: i to capture eagerly.
• Rebinding an outer variable without nonlocal — Python creates a new local and the closure appears broken.
• Leaking large objects in closure cells — a captured DataFrame keeps memory alive; use weakref or explicit del.