Iterator protocol in Python - Step-by-Step Execution

Concept Flow - Iterator protocol

Create iterable object

↓

Call iter() to get iterator

↓

Call next() on iterator

↓

Yield next item if available

↓

StopIteration raised when done

↓

End

The iterator protocol means an object can be looped over by calling iter() to get an iterator, then repeatedly calling next() until no items remain.

Execution Sample

Python

numbers = [10, 20, 30]
it = iter(numbers)
print(next(it))
print(next(it))
print(next(it))

This code creates an iterator from a list and prints each item one by one using next().

Execution Table

Step	Action	Iterator State	next() Result	Output
1	Create list 'numbers'	N/A	N/A	N/A
2	Call iter(numbers) to get iterator 'it'	it points before first element	N/A	N/A
3	Call next(it)	it moves to first element	10	10
4	Call next(it)	it moves to second element	20	20
5	Call next(it)	it moves to third element	30	30
6	Call next(it) again	No more elements	StopIteration raised	Program stops if not caught

💡 StopIteration raised because iterator has no more elements

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
numbers	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]
it (iterator position)	N/A	Before first element	At 10	At 20	At 30	End (no more elements)

Key Moments - 3 Insights

Why does calling next() after the last item cause an error?

Because the iterator has no more items, next() raises StopIteration to signal the end (see step 6 in execution_table).

Is the original list 'numbers' changed by iter() or next()?

No, the list stays the same; only the iterator's position changes (see variable_tracker for 'numbers' and 'it').

What does iter() actually do?

iter() returns an iterator object that keeps track of the current position for next() calls (step 2 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of next(it) at step 4?

A 20

B 30

C 10

D StopIteration

💡 Hint

Check the 'next() Result' column at step 4 in execution_table.

At which step does the iterator reach the end and raise StopIteration?

A Step 3

B Step 5

C Step 6

D Step 2

💡 Hint

Look for 'StopIteration raised' in the 'next() Result' column.

If we call next(it) only twice, what will be the iterator's position after that?

A At 10

B At 20

C Before first element

D At 30

💡 Hint

Check variable_tracker for 'it' after step 4.

Concept Snapshot

Iterator protocol in Python:
- Use iter(obj) to get an iterator.
- Use next(iterator) to get next item.
- When no items left, next() raises StopIteration.
- Original object stays unchanged.
- Enables looping over objects step-by-step.

Full Transcript

The iterator protocol in Python allows an object to be looped over by first calling iter() to get an iterator. This iterator remembers the current position. Each call to next() returns the next item and moves the position forward. When there are no more items, next() raises StopIteration to signal the end. The original object does not change during this process. This protocol is how Python loops like for work behind the scenes.