String immutability in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new string by replacing the first character.

Python

text = "hello"
new_text = [1] + text[1:]
print(new_text)

Drag options to blanks, or click blank then click option'

A"j"

Bj

C'j'

D"hello"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a character without quotes causes an error.

Trying to assign directly to a string index.

Strings must be enclosed in quotes. Using 'j' creates a new string starting with 'j'.

2fill in blank

medium

Complete the code to show that strings cannot be changed by assignment.

Python

text = "hello"
try:
    text[0] = [1]
except TypeError:
    print("Cannot change string characters directly")

Drag options to blanks, or click blank then click option'

A'j'

B"j"

Cj

D"hello"

Attempts:

3 left

💡 Hint

Common Mistakes

Using an unquoted character causes a NameError.

Trying to assign to string index without quotes.

Assigning a new character to a string index requires a string in quotes.

3fill in blank

hard

Fix the error in the code that tries to change a string character.

Python

word = "world"
word = [1] + word[1:]
print(word)

Drag options to blanks, or click blank then click option'

A"W"

B'W'

CW

D"world"

Attempts:

3 left

💡 Hint

Common Mistakes

Using an unquoted character causes an error.

Trying to assign directly to string index.

To create a new string with a changed first letter, use a quoted character and concatenate.

4fill in blank

hard

Fill both blanks to create a new string with the last character replaced.

Python

word = "python"
new_word = word[:[1]] + [2]
print(new_word)

Drag options to blanks, or click blank then click option'

A-1

B"!"

C5

D'!'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 6 as slice index includes the last character.

Using unquoted character causes error.

Use 5 to slice up to the last character (index 5), and '!' as the new last character.

5fill in blank

hard

Fill all three blanks to replace the middle character in a string.

Python

word = "apple"
new_word = word[:[1]] + [2] + word[[3]:]
print(new_word)

Drag options to blanks, or click blank then click option'

A2

B"i"

C3

D'i'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong slice indexes.

Using unquoted replacement character.

Slice before index 2, add 'i' as a string, then slice from index 3 to end.

Practice

1. What does it mean that strings in Python are immutable?

easy

A. Strings are stored as numbers internally.

B. You can change parts of a string directly.

C. Strings can be resized but not changed.

D. You cannot change a string after it is created.

Solution

1. Step 1: Understand the meaning of immutable

   Immutable means something cannot be changed after it is made.

2. Step 2: Apply this to strings in Python

   Strings cannot be changed once created; you must make a new string to change text.

3. Final Answer:

   You cannot change a string after it is created. -> Option D

4. Quick Check:

   Immutable = cannot change [OK]

Hint: Immutable means no changes allowed after creation [OK]

Common Mistakes:

• Thinking strings can be changed like lists
• Confusing immutability with fixed size
• Believing strings store numbers only

2. Which of the following is the correct way to create a new string by changing the first letter of word = 'hello' to uppercase?

easy

A. word[0] = 'H'

B. word = 'H' + word[1:]

C. word.upper()[0]

D. word.change(0, 'H')

Solution

1. Step 1: Recognize string immutability

   You cannot assign to a string index like word[0] = 'H'.

2. Step 2: Create a new string combining parts

   Use slicing and concatenation: 'H' + word[1:] makes a new string with first letter changed.

3. Final Answer:

   word = 'H' + word[1:] -> Option B

4. Quick Check:

   Use slicing + concat to change string [OK]

Hint: Use slicing and + to build new strings [OK]

Common Mistakes:

• Trying to assign directly to string index
• Using non-existent string methods like change()
• Misusing upper() which returns full uppercase

3. What will be the output of this code?

text = 'cat'
text[0] = 'b'
print(text)

medium

A. TypeError

B. cat

C. bat

D. SyntaxError

Solution

1. Step 1: Identify the operation on string index

   Trying to assign text[0] = 'b' attempts to change a character in the string.

2. Step 2: Recall string immutability rules

   Strings cannot be changed by index assignment; Python raises a TypeError.

3. Final Answer:

   TypeError -> Option A

4. Quick Check:

   Index assignment on string = TypeError [OK]

Hint: Assigning to string index causes TypeError [OK]

Common Mistakes:

• Expecting string to change without error
• Confusing TypeError with SyntaxError
• Thinking print shows changed string

4. The code below tries to replace the first character of word with 'J'. What is wrong and how to fix it?

word = 'python'
word[0] = 'J'
print(word)

medium

A. Strings are immutable; fix by creating a new string: word = 'J' + word[1:]

B. Syntax error; fix by adding parentheses around 'J'

C. Use word.replace(0, 'J') to fix

D. No error; code works fine

Solution

1. Step 1: Identify the error cause

   Assigning to word[0] is invalid because strings cannot be changed by index.

2. Step 2: Fix by creating a new string

   Build a new string with 'J' plus the rest: word = 'J' + word[1:].

3. Final Answer:

   Strings are immutable; fix by creating a new string: word = 'J' + word[1:] -> Option A

4. Quick Check:

   Fix immutability by new string creation [OK]

Hint: Fix by slicing and concatenating new string [OK]

Common Mistakes:

• Trying to assign directly to string index
• Using replace() incorrectly for index change
• Assuming no error occurs

5. Given a string s = 'banana', how can you create a new string where all 'a' characters are replaced by 'o' without modifying s directly?

hard

A. s = s[0] + 'o' + s[2:]

B. s[1] = 'o'

C. s.replace('a', 'o')

D. s.change('a', 'o')

Solution

1. Step 1: Understand string immutability

   You cannot change characters by index; must create a new string.

2. Step 2: Use string method to replace characters

   s.replace('a', 'o') returns a new string with all 'a' replaced by 'o'.

3. Final Answer:

   s.replace('a', 'o') -> Option C

4. Quick Check:

   Use replace() to get new string with changes [OK]

Hint: Use replace() to get new string with changes [OK]

Common Mistakes:

• Trying to assign to string index
• Using non-existent change() method
• Changing only one character manually