reversed() function in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the reversed() function do in Python?

It returns an iterator that accesses the given sequence in reverse order without changing the original sequence.

beginner

Can reversed() be used on any iterable in Python?

No, reversed() works only on sequences that support indexing and have a defined length, like lists, tuples, and strings.

beginner

How do you convert the result of reversed() into a list?

Use the list() function around reversed(), like list(reversed(my_list)).

beginner

What is the output of list(reversed('hello'))?

['o', 'l', 'l', 'e', 'h'] - the characters of the string 'hello' in reverse order as a list.

beginner

Does reversed() modify the original sequence?

No, it does not change the original sequence; it only provides a reversed iterator.

Which of these can reversed() be used on?

A A dictionary

B A list

C A set

D A generator

reversed() works on sequences like lists, not on dictionaries, sets, or generators.

What type does reversed() return?

A A list

B A string

C A tuple

D An iterator

reversed() returns an iterator that you can loop over or convert to a list.

How do you get a reversed list from a tuple t?

A list(reversed(t))

B reversed(list(t))

C t[::-1]

D All of the above

Only list(reversed(t)) produces a reversed list from a tuple t. reversed(list(t)) returns an iterator and t[::-1] returns a reversed tuple.

What happens if you try reversed() on a set?

A It returns a reversed iterator

B It returns the set unchanged

C It raises a TypeError

D It converts the set to a list automatically

reversed() requires a sequence with indexing; sets do not support this, so it raises an error.

Which of these is a correct way to print characters of a string in reverse?

A Both B and C

B print('abc'[::-1])

C print(list(reversed('abc')))

D print(reversed('abc'))

Slicing with [::-1] and converting reversed() to list both print reversed characters.

Explain how the reversed() function works and when you would use it.

Think about how you might want to read a list or string backwards without changing it.

Describe the difference between using reversed() and slicing with [::-1] to reverse a sequence.

Consider the type of result and performance differences.

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()