Lambda functions, iterators and generators

A lambda is an anonymous function made of a single expression, such as lambda x: x * 2. It is often passed to map, filter and sorted(key=...): filter keeps items that pass a test, map transforms each item, and the sort key decides the ordering.

An iterable can be looped over; an iterator is an object with __iter__ and __next__ that produces one item per next() call and raises StopIteration when finished. next(it, default) returns the default instead of raising. A class becomes an iterator by defining __iter__ (returning self) and __next__.

A generator is a function that uses yield. Calling it returns a generator object without running the body; each next() runs the code up to the next yield, so work is done lazily and memory stays small. yield from hands control to another iterable. A generator expression such as (x * x for x in range(3)) works the same way, and either kind can be consumed only once.

Exam trap: lambdas created in a loop capture the variable, not its value, so [lambda: i for i in range(3)] all return the last i.