reversed() function in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reversed() on a string

What is the output of this Python code?

Python

s = 'hello'
result = ''.join(reversed(s))
print(result)

A Error: reversed() cannot be used on strings

B olleh

C ['o', 'l', 'l', 'e', 'h']

D hello

💡 Hint

reversed() returns an iterator that goes through the input backwards.

The reversed() function returns an iterator that yields characters from the string in reverse order. Joining these characters produces the reversed string.

❓ Predict Output

intermediate

Using reversed() on a list

What is the output of this code snippet?

Python

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

A [1, 2, 3, 4]

B [1, 4, 2, 3]

C [4, 3, 2, 1]

D Error: reversed() only works on strings

💡 Hint

reversed() works on any sequence, including lists.

reversed(lst) returns an iterator that goes through the list backwards. Converting it to a list shows the reversed order.

❓ Predict Output

advanced

Output of reversed() on a dictionary keys view

What happens when you run this code?

Python

d = {'a': 1, 'b': 2, 'c': 3}
print(list(reversed(d.keys())))

A TypeError: 'dict_keys' object is not reversible

B ['a', 'b', 'c']

C ['c', 'b', 'a']

D ['a', 'c', 'b']

💡 Hint

Check if dict_keys supports reversed() directly.

dict_keys objects do not support reversed() because they are not sequences and do not implement __reversed__.

❓ Predict Output

advanced

Reversing a range object

What is the output of this code?

Python

r = range(1, 5)
print(list(reversed(r)))

A [4, 3, 2, 1]

B [1, 2, 3, 4]

C TypeError: 'range' object is not reversible

D [5, 4, 3, 2]

💡 Hint

range objects support reversed() because they are sequences.

range(1, 5) generates numbers 1 to 4. reversed(range(1,5)) iterates backwards from 4 to 1.

🧠 Conceptual

expert

Behavior of reversed() on custom iterable

Consider this class: class CountDown: def __init__(self, start): self.start = start def __iter__(self): n = self.start while n > 0: yield n n -= 1 What happens if you call reversed(CountDown(3))?

A [] (empty list)

B [3, 2, 1]

C [1, 2, 3]

D TypeError: object is not reversible

💡 Hint

reversed() requires __reversed__ or __len__ and __getitem__ methods.

The CountDown class only implements __iter__, not __reversed__ or __getitem__, so reversed() cannot reverse it and raises TypeError.

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()