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Mutable vs Immutable Data Types

In Python, every value we store in a variable has a data type. These data types are divided into two categories: mutable and immutable, depending on whether their values can be changed after creation.

Mutable Data Types

Mutable means changeable. If a data type is mutable, it means you can add, remove, or modify its elements without creating a new object in memory. When you change a mutable object, its memory address (ID) stays the same.

Common mutable data types:

  • list
  • dict
  • set
  • bytearray

Example:

# Example of a mutable data type (list)
fruits = ["apple", "banana", "cherry"]
print(id(fruits))  # Memory address before modification
 
fruits.append("mango")  # Adding a new item
print(fruits)
print(id(fruits))  # Same memory address → same object

Output:

140394891456640
['apple', 'banana', 'cherry', 'mango']
140394891456640

Explanation: The memory address remains the same even after adding a new element. This means the original list was changed directly, not replaced.

Immutable Data Types

Immutable means unchangeable. If a data type is immutable, it means its value cannot be changed once created. If you try to modify it, Python will create a new object instead of changing the existing one. As a result, its memory address (ID) changes.

Common immutable data types:

  • int
  • float
  • complex
  • str
  • tuple
  • bool
  • bytes

Example:

# Example of an immutable data type (string)
name = "Harry"
print(id(name))  # Memory address before modification
 
name = name + " Potter"  # Concatenating creates a new string
print(name)
print(id(name))  # Different memory address → new object created

Output:

140394891763824
Harry Potter
140394891765008

Explanation: Here, a new string "Harry Potter" was created in memory instead of modifying the old one. That’s why the memory address changed.

Summary Table

Category Data Type(s) Mutable Immutable
Numeric int, float, complex No Yes
Text str No Yes
Boolean bool No Yes
Sequence list, tuple, range Yes (list) Yes (tuple, range)
Mapped dict Yes No
Set set Yes No
Binary bytes, bytearray, memoryview Yes (bytearray) Yes (bytes)
None Type NoneType No Yes

Notes

  1. Mutable means the value can change without creating a new object.
  2. Immutable means the value cannot change; a new object is created when modified.
  3. Mutable objects are useful when you need to update data frequently (like lists or dictionaries).
  4. Immutable objects help prevent accidental changes and make your code more predictable.
Data TypesNumbers