Iterable and Iterator Protocol

Scenario: A for loop somehow knows how to walk lists, files, ranges, and custom objects.

New words, in plain English

• Iterable - an object iter() can ask for an iterator
• Iterator - an object next() asks for one value at a time
• StopIteration - the signal that no values remain
• Lazy - computed only when requested

A for loop calls iter, repeatedly calls next, then stops on StopIteration. Iterators are consumed; create a fresh one to start over.

Analogy: A Python program is like a clear set of instructions for a careful helper: names label things, indentation groups steps, and errors explain where the helper became confused.

A small, real example

values = iter([10, 20])
print(next(values))
print(next(values))
# next(values) would now raise StopIteration

Tip: Most code should use for; knowing the protocol explains why it works.

Goal: Practice this idea in the py-generators challenge at /labs/python. Fix the broken program, run it yourself, then use check to prove the real end state.