A string in Python is immutable, which means its contents cannot be changed after the string has been created.

Once a string is assigned to a variable, individual characters cannot be modified, removed, or replaced directly.

If you want to change a string, Python creates a new string instead of modifying the existing one.

String immutability makes strings safer to use, allows Python to optimize memory usage, and prevents accidental modification of text.

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Syntax #

string_name[index]

You can read characters using indexing, but you cannot modify them.

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Example 1: Reading a Character #

main.py

language = "Python"

print(language[0])

Output #

P

Explanation #

• Indexing can be used to read characters.
• The character at index 0 is P.
• Reading characters does not modify the string.

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Example 2: Attempting to Modify a Character #

main.py

language = "Python"

language[0] = "J"

Output #

TypeError: 'str' object does not support item assignment

Explanation #

• Python does not allow characters in a string to be changed.
• Strings are immutable.
• Attempting to assign a new character produces a TypeError.

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Example 3: Creating a New String #

main.py

language = "Python"

language = "Jython"

print(language)

Output #

Jython

Explanation #

• The original string is not modified.
• Instead, the variable is assigned a completely new string.
• The variable now refers to the new string.

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Example 4: Using Concatenation to Create a New String #

main.py

word = "Python"

new_word = "J" + word[1:]

print(new_word)

Output #

Jython

Explanation #

• word[1:] returns "ython".
• "J" is concatenated with "ython".
• A new string "Jython" is created.
• The original string remains "Python".

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Example 5: Original String Remains Unchanged #

main.py

text = "Python"

copy = text

copy = "Java"

print(text)
print(copy)

Output #

Python
Java

Explanation #

• Initially, both variables refer to the same string.
• Reassigning copy does not affect text.
• The original string remains unchanged.

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Example 6: Replacing a Character Using Slicing #

main.py

word = "Python"

new_word = word[:1] + "A" + word[2:]

print(new_word)

Output #

PAthon

Explanation #

• word[:1] returns "P".
• word[2:] returns "thon".
• Concatenating the parts creates a new string.
• The original string is not modified.

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Example 7: User Input #

main.py

name = input("Enter your name: ")

name = name.upper()

print(name)

Sample Input #

alice

Output #

ALICE

Explanation #

• upper() returns a new uppercase string.
• The original input string is not modified.
• The variable name is updated to reference the new string.

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Example 8: Checking Object Identity #

main.py

text = "Python"

print(id(text))

text = text + " Programming"

print(id(text))

Output #

140437761...
140437845...

Explanation #

• id() returns the memory identity of an object.
• After concatenation, Python creates a new string.
• The memory identity changes because a new object is created.

Note: The exact numbers returned by id() vary from one execution to another.

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Example 9: Strings Returned by Methods #

main.py

text = "python"

uppercase = text.upper()

print(text)
print(uppercase)

Output #

python
PYTHON

Explanation #

• upper() does not modify the original string.
• It returns a new uppercase string.
• The original string remains unchanged.

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Example 10: Verifying Immutability #

main.py

word = "Computer"

print(word)

new_word = word.replace("C", "L")

print(new_word)
print(word)

Output #

Computer
Lomputer
Computer

Explanation #

• replace() creates a new string.
• The original string is not modified.
• Both strings exist independently.

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Common Mistakes #

1. Trying to Modify a Character #

Incorrect

word = "Python"

word[1] = "A"

Output #

TypeError: 'str' object does not support item assignment

Reason

Strings are immutable.
Individual characters cannot be changed.

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2. Assuming String Methods Modify the Original String #

Incorrect

text = "python"

text.upper()

print(text)

Output #

python

Reason

upper() returns a new string.
It does not modify the original one.

Correct

text = text.upper()

print(text)

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3. Assuming Concatenation Changes the Original String #

Incorrect

text = "Hello"

text + " World"

print(text)

Output #

Hello

Reason

Concatenation creates a new string.
The original string remains unchanged.

Correct

text = text + " World"

print(text)

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4. Confusing Strings with Lists #

Incorrect

text = "Python"

text[2] = "X"

Output #

TypeError: 'str' object does not support item assignment

Reason

Lists are mutable, but strings are immutable.

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Best Practices #

• Treat strings as read-only objects.
• Use slicing and concatenation to create modified versions of strings.
• Store the result returned by string methods.
• Remember that every modification produces a new string.
• Avoid assuming that string methods modify the original string.

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Key Points to Remember #

• Strings in Python are immutable.
• Individual characters cannot be modified after a string is created.
• String methods return new strings instead of modifying existing ones.
• Concatenation creates a new string.
• Slicing creates a new string.
• Reassigning a variable changes what it refers to, not the original string.
• Attempting to modify a character directly raises a TypeError.