String immutability in Python - Step-by-Step Execution

Concept Flow - String immutability

Create string s = "hello"

↓

Try to change s[0

↓

Error: strings cannot be changed

↓

Create new string s = "Hello"

↓

Use new string s

↓

END

Strings cannot be changed once created. To modify, you make a new string instead.

Execution Sample

Python

s = "hello"
s[0] = 'H'  # Error
s = "Hello"  # New string
print(s)

This code shows that trying to change a character in a string causes an error, so we create a new string instead.

Execution Table

Step	Action	Code Line	Result	Notes
1	Create string s	s = "hello"	s = "hello"	String s holds 'hello'
2	Try to change s[0]	s[0] = 'H'	TypeError	Strings are immutable, error raised
3	Create new string s	s = "Hello"	s = "Hello"	New string assigned to s
4	Print s	print(s)	Hello	Output is the new string

💡 Execution stops after printing because the string was replaced, not modified.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
s	undefined	"hello"	Error (no change)	"Hello"	"Hello"

Key Moments - 2 Insights

Why does s[0] = 'H' cause an error?

Because strings cannot be changed after creation, as shown in step 2 of the execution_table where a TypeError occurs.

How do we change the content of a string variable?

We create a new string and assign it to the variable, like in step 3 where s is assigned "Hello".

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of s after step 1?

A "hello"

B "Hello"

C Error

D undefined

💡 Hint

Check the 'Result' column in row for step 1 in execution_table.

At which step does the program raise an error due to string immutability?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Result' column in execution_table for the step where TypeError appears.

If we want to change the first letter of s to uppercase, what should we do?

A Use s.append('H')

B Assign a new string with the change to s

C Change s[0] directly

D Delete s and create a list

💡 Hint

Refer to step 3 in execution_table where a new string is assigned to s.

Concept Snapshot

String immutability in Python:
- Strings cannot be changed once created.
- Trying to change a character causes an error.
- To modify, create a new string and assign it.
- Example: s = "hello"; s = "Hello"
- Always remember: strings are fixed after creation.

Full Transcript

This visual execution shows that strings in Python cannot be changed after they are created. When we try to change a character in the string s, Python raises a TypeError because strings are immutable. To change the content, we create a new string and assign it to the variable s. The execution table traces each step: creating the string, attempting to change it (which causes an error), creating a new string, and printing the result. The variable tracker shows how s changes from undefined to 'hello', then an error occurs, and finally s holds 'Hello'. This helps beginners understand that strings cannot be changed in place but must be replaced with new strings.

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