Question 1

What signals exhaustion in the iterator protocol

Accepted Answer

The iterator protocol uses StopIteration as the signal that the sequence is exhausted. A for loop catches this exception automatically and exits cleanly - you never see it as an error in normal iteration. Returning None would be ambiguous because None is a valid value a generator might legitimately yield.

Question 2

iter() and next() on a list

Accepted Answer

iter() returns an iterator that tracks a position inside the list. Each call to next() advances that position by one and returns the current value. The iterator starts at index 0, so the first next() gives 10 and the second gives 20. The original list is unchanged - only the iterator's cursor moves.

Question 3

Basic generator function output

Accepted Answer

A generator function pauses at each yield and resumes from that point on the next iteration. Here the loop runs while i < 3, yielding i=0, i=1, i=2, then the while condition fails and StopIteration is raised. Because the range is 0 to n-1 (not inclusive of n), the value 3 is never yielded.