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.