Every object in Python is either mutable or immutable.

• A mutable object can be modified after it is created.
• An immutable object cannot be modified after it is created.

Understanding this concept is important because it explains why some objects can be changed directly while others require a new object to be created.

Some common immutable objects are:

• int
• float
• bool
• str
• tuple

Some common mutable objects are:

• list
• dict
• set

---

Example 1: Immutable Integer #

main.py

number = 100

print(id(number))

number = 200

print(id(number))

Sample Output #

140516082641360
140516082644560

Explanation #

• Initially, number refers to the integer object 100.
• After assigning 200, Python creates another integer object.
• The variable now refers to the new object.
• Integers are immutable, so the original object is never modified.

---

Example 2: Immutable String #

main.py

text = "Python"

print(id(text))

text = text + " Programming"

print(text)
print(id(text))

Sample Output #

Python Programming
140516082751824

Explanation #

• Strings cannot be modified after creation.
• The expression text + " Programming" creates a new string.
• The variable now refers to the new string object.
• The original string remains unchanged.

---

Example 3: Mutable List #

main.py

numbers = [10, 20, 30]

print(id(numbers))

numbers.append(40)

print(numbers)
print(id(numbers))

Output #

[10, 20, 30, 40]
140516082913344

Explanation #

• numbers refers to a list.
• The append() method modifies the existing list.
• The identity of the list remains the same.
• Lists are mutable objects.

---

Example 4: Modifying a List Element #

main.py

numbers = [10, 20, 30]

numbers[1] = 50

print(numbers)

Output #

[10, 50, 30]

Explanation #

• Lists allow individual elements to be modified.
• The second element changes from 20 to 50.
• The list object itself remains the same.

---

Example 5: Tuples are Immutable #

main.py

numbers = (10, 20, 30)

print(numbers)

Output #

(10, 20, 30)

Explanation #

• Tuples are immutable collections.
• Once created, their elements cannot be modified.
• To change a tuple, a new tuple must be created.

---

Example 6: Attempting to Modify a Tuple #

main.py

numbers = (10, 20, 30)

numbers[1] = 50

Output #

TypeError: 'tuple' object does not support item assignment

Explanation #

• Python does not allow tuple elements to be changed.
• Attempting to assign a new value raises a TypeError.

---

Example 7: Assignment with Mutable Objects #

main.py

list1 = [10, 20, 30]

list2 = list1

list2.append(40)

print(list1)
print(list2)

Output #

[10, 20, 30, 40]
[10, 20, 30, 40]

Explanation #

• Both variables refer to the same list object.
• Modifying the list through list2 also affects list1.
• This happens because lists are mutable.

---

Example 8: Creating an Independent Copy #

main.py

list1 = [10, 20, 30]

list2 = list1.copy()

list2.append(40)

print(list1)
print(list2)

Output #

[10, 20, 30]
[10, 20, 30, 40]

Explanation #

• copy() creates a new list object.
• The two variables now refer to different lists.
• Modifying one list does not affect the other.

---

Example 9: Dictionary is Mutable #

main.py

student = {
    "name": "Alice",
    "age": 20
}

student["age"] = 21

print(student)

Output #

{'name': 'Alice', 'age': 21}

Explanation #

• Dictionaries are mutable.
• Existing values can be updated.
• New key-value pairs can also be added later.

---

Example 10: Set is Mutable #

main.py

numbers = {10, 20, 30}

numbers.add(40)

print(numbers)

Sample Output #

{10, 20, 30, 40}

Explanation #

• Sets are mutable collections.
• The add() method inserts a new element into the existing set.
• The set object itself is modified.

---

Common Mistakes #

1. Assuming Strings Can Be Modified #

Incorrect

text = "Python"

text[0] = "J"

Output #

TypeError: 'str' object does not support item assignment

Reason

Strings are immutable.

Correct

text = "Jython"

---

2. Assuming Assignment Creates a Copy #

Incorrect

list1 = [1, 2, 3]

list2 = list1

Reason

Both variables refer to the same list object.

Correct

list2 = list1.copy()

---

3. Trying to Modify a Tuple #

Incorrect

values = (1, 2, 3)

values[0] = 10

Output #

TypeError: 'tuple' object does not support item assignment

Reason

Tuples are immutable.

---

4. Assuming All Collections are Mutable #

Incorrect Assumption

tuple_data = (10, 20)

tuple_data[1] = 30

Reason

Lists, dictionaries, and sets are mutable, but tuples are immutable.

---

Best Practices #

• Use immutable objects when values should never change.
• Use mutable collections when data needs to be updated.
• Create a copy of mutable objects when independent data is required.
• Understand the difference between assignment and copying.
• Remember that strings and tuples cannot be modified after creation.

---

Key Points to Remember #

• Objects in Python are either mutable or immutable.
• Mutable objects can be modified after creation.
• Immutable objects cannot be modified after creation.
• Common immutable types include int, float, bool, str, and tuple.
• Common mutable types include list, dict, and set.
• Assignment copies a reference, not the object itself.
• Use .copy() to create an independent copy of mutable collections such as lists.
• Understanding mutability helps prevent unexpected changes in programs.