reversed() function in Python - Step-by-Step Execution

Concept Flow - reversed() function

Input iterable

↓

Call reversed()

↓

Create reverse iterator

↓

Iterate over reversed elements

↓

Output elements in reverse order

↓

End iteration

The reversed() function takes an iterable and returns an iterator that yields elements in reverse order until all elements are exhausted.

Execution Sample

Python

items = [1, 2, 3]
rev_iter = reversed(items)
for x in rev_iter:
    print(x)

This code reverses the list [1, 2, 3] and prints each element in reverse order.

Execution Table

Step	Action	Iterator State	Output
1	Create reversed iterator from [1, 2, 3]	[3, 2, 1]
2	Get next element from iterator	[2, 1]	3
3	Get next element from iterator	[1]	2
4	Get next element from iterator	[]	1
5	Get next element from iterator	[]	StopIteration raised

💡 Iterator exhausted, no more elements to return

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
items	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]
rev_iter	None	Iterator over [3, 2, 1]	Iterator over [2, 1]	Iterator over [1]	Iterator empty	Iterator exhausted

Key Moments - 3 Insights

Why does reversed() return an iterator and not a list?

reversed() returns an iterator to save memory and allow lazy evaluation, as shown in execution_table steps 1-5 where elements are produced one by one.

What happens if you try to get next element after the iterator is exhausted?

As shown in step 5 of execution_table, a StopIteration exception is raised indicating no more elements are available.

Can reversed() be used on any iterable?

No, reversed() requires a sequence (like list, tuple, or string) that supports __reversed__ or __len__ and __getitem__, otherwise it raises an error.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at Step 3?

A 3

B 1

C 2

D StopIteration

💡 Hint

Check the Output column at Step 3 in execution_table

At which step does the iterator become empty?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the Iterator State column to see when it becomes []

If the input list was empty, what would happen when calling reversed()?

A StopIteration immediately on next()

B Iterator with elements

C Error raised

D Output the original list

💡 Hint

Consider what happens when reversed() is called on an empty sequence and check exit_note

Concept Snapshot

reversed(iterable)
- Returns an iterator that yields elements in reverse order
- Works on sequences supporting __reversed__ or __getitem__
- Does not create a reversed list, saves memory
- Raises StopIteration when done
- Use in for-loops or with next() to access reversed elements

Full Transcript

The reversed() function takes a sequence like a list and returns an iterator that produces elements from the end to the start. When you call reversed() on [1, 2, 3], it creates an iterator starting at 3. Each call to next() returns the next element backward: 3, then 2, then 1. After all elements are returned, next() raises StopIteration to signal the end. The original list remains unchanged. reversed() is efficient because it does not copy the list but yields elements one by one. It only works on sequences that support reverse iteration. If you try reversed() on an empty list, the iterator is empty and raises StopIteration immediately.