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.

Practice

1. What does the reversed() function do in Python?

easy

A. It sorts the items of a sequence in ascending order.

B. It returns an iterator that goes through the items of a sequence backwards.

C. It removes duplicate items from a sequence.

D. It changes the original sequence to its reversed form.

Solution

1. Step 1: Understand the purpose of reversed()

   The reversed() function returns an iterator that accesses the elements of a sequence in reverse order.

2. Step 2: Check if original sequence changes

   The original sequence remains unchanged; reversed() only provides a way to loop backwards.

3. Final Answer:

   It returns an iterator that goes through the items of a sequence backwards. -> Option B

4. Quick Check:

   reversed() returns reversed iterator [OK]

Hint: Remember: reversed() does not change original, just reads backwards [OK]

Common Mistakes:

• Thinking reversed() sorts the sequence
• Assuming reversed() modifies the original sequence
• Confusing reversed() with removing duplicates

2. Which of the following is the correct way to use reversed() to print characters of a string backwards?

easy

A. for ch in reversed('hello'): print(ch)

B. for ch in reverse('hello'): print(ch)

C. for ch in 'hello'.reversed(): print(ch)

D. for ch in reversed['hello']: print(ch)

Solution

1. Step 1: Identify correct function syntax

   reversed() is a built-in function called with parentheses and a sequence inside, like reversed('hello').

2. Step 2: Check loop syntax

   The for loop correctly iterates over the reversed iterator returned by reversed().

3. Final Answer:

   for ch in reversed('hello'): print(ch) -> Option A

4. Quick Check:

   Use reversed() with parentheses and a sequence [OK]

Hint: Use reversed() with parentheses, not square brackets or dot calls [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Calling reversed as a method on string
• Typing reverse instead of reversed

3. What is the output of this code?

print(list(reversed([1, 2, 3, 4])))

medium

A. Error: reversed() cannot be used on lists

B. [1, 2, 3, 4]

C. [4, 3, 2, 1]

D. [1, 4, 3, 2]

Solution

1. Step 1: Apply reversed() to the list

   reversed([1, 2, 3, 4]) returns an iterator that goes through the list backwards: 4, 3, 2, 1.

2. Step 2: Convert iterator to list

   Using list() on the reversed iterator collects all items into a new list in reversed order.

3. Final Answer:

   [4, 3, 2, 1] -> Option C

4. Quick Check:

   list(reversed([1,2,3,4])) = [4,3,2,1] [OK]

Hint: Wrap reversed() with list() to see reversed items as a list [OK]

Common Mistakes:

• Expecting reversed() to return a list directly
• Confusing reversed() with sort()
• Thinking reversed() modifies original list

4. What is wrong with this code?

my_str = 'abc'
rev_str = reversed(my_str)
print(rev_str)

medium

A. It prints a reversed iterator object, not the reversed string.

B. It prints a reversed string directly.

C. It causes a syntax error because reversed() needs a list.

D. It modifies my_str permanently.

Solution

1. Step 1: Understand what reversed() returns

   reversed(my_str) returns an iterator, not a string.

2. Step 2: Printing the iterator shows its object info

   Printing rev_str directly prints something like <reversed object at ...>, not the reversed characters.

3. Final Answer:

   It prints a reversed iterator object, not the reversed string. -> Option A

4. Quick Check:

   reversed() returns iterator, print shows object [OK]

Hint: Convert reversed() result to list or string before printing [OK]

Common Mistakes:

• Expecting reversed() to return a string
• Trying to print reversed() result directly
• Thinking reversed() modifies original string

5. How can you use reversed() to create a new string that is the reverse of the original string s = 'Python'?

hard

A. new_s = s.reverse()

B. new_s = reversed(s)

C. new_s = s[::-1]

D. new_s = ''.join(reversed(s))

Solution

1. Step 1: Use reversed() to get reversed iterator

   reversed(s) returns an iterator over characters of s in reverse order.

2. Step 2: Join characters into a new string

   ''.join(reversed(s)) combines the reversed characters into a new string.

3. Final Answer:

   new_s = ''.join(reversed(s)) -> Option D

4. Quick Check:

   Use join() with reversed() to build reversed string [OK]

Hint: Use ''.join(reversed(s)) to reverse strings easily [OK]

Common Mistakes:

• Assigning reversed(s) directly to string variable
• Using s.reverse() which is invalid for strings
• Confusing slicing with reversed()