String immutability in Python - Step-by-Step Execution

Concept Flow - String immutability

Create string s = "hello"

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Try to change s[0

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Error: strings cannot be changed

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Create new string s = "Hello"

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Use new string s

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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.