Python - Immutable Data Structures

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The Python Immutable data structures are the data structures that once created, cannot be changed. This means that any attempt to modify the data structure will result in a new instance being created rather than altering the original. Immutable data structures are useful for ensuring that data remains constant throughout the execution of a program which can help prevent bugs and make code easier to understand and maintain.

Before proceeding deep into this topic let's have a quick recall of what is datastructure? The Data structures are specialized formats for organizing, processing, retrieving and storing data. They define how data is arranged in memory and how operations such as accessing, inserting, deleting and updating can be performed efficiently.

Different Immutable Data Structures in Python

Immutable data structures are essential in Python for their stability, thread-safety and ease of use. Here are the different immutable data structures in Python −

• Tuples: These are the ordered collections of items that cannot be changed after their creation. They can contain mixed data types and are useful for representing fixed collections of related items.
• Strings: These Data structures are sequences of characters and are immutable. Any operation that modifies a string will create a new string.
• Frozensets: These are immutable versions of sets. Unlike regular sets, frozensets do not allow modification after creation.
• Named Tuples: These are a subclass of tuples with named fields which provide more readable and self-documenting code. They are immutable like regular tuples.

Now, let's proceed about the each Immutable data structures in detail.

Tuples

Tuples in Python are immutable sequences of elements which means once created, they cannot be modified. They are defined using parentheses '()' and can hold a collection of items such as numbers, strings and even other tuples.

Creating Tuples

Tuples are created using parentheses '()' and elements separated by commas ','. Even tuples with a single element require a trailing comma to distinguish them from grouped expressions.

Following is the example of creating a tuple by assigning parentheses '()' to a variable −

empty_tuple = ()
single_element_tuple = (5,)  # Note the comma after the single element
print("Single element tuple:", single_element_tuple)
multi_element_tuple = (1, 2, 'Tutorialspoint', 3.14)
print("Multi elements tuple:", multi_element_tuple)
nested_tuple = (1, (2, 3), 'Learning')
print("Nested tuple:", nested_tuple)

On executing the above code we will get the following output −

Single element tuple: (5,)
Multi elements tuple: (1, 2, 'Tutorialspoint', 3.14)
Nested tuple: (1, (2, 3), 'Learning')

Understanding Tuple Immutability in Python

Here we are going understand the immutability of the tuples in python. Below is the example −

# Define a tuple
my_tuple = (1, 2, 3, 'hello')
# Attempt to modify an element (which is not possible with tuples)
# This will raise a TypeError
try:
   my_tuple[0] = 10
except TypeError as e:
   print(f"Error: {e}")
# Even trying to append or extend a tuple will result in an error
try:
   my_tuple.append(4)
except AttributeError as e:
   print(f"Error: {e}")
# Trying to reassign the entire tuple to a new value is also not allowed
try:
   my_tuple = (4, 5, 6)
except TypeError as e:
   print(f"Error: {e}")
print("Original tuple:", my_tuple)

On executing the above code we will get the following output −

Error: 'tuple' object does not support item assignment
Error: 'tuple' object has no attribute 'append'
Original tuple: (4, 5, 6)

Strings

Strings in Python are sequences of characters which are used to represent and manipulate textual data. They are enclosed within either single quotes ' or double quotes " with the option to use triple quotes """ for multi-line strings. Key characteristics include immutability which means once created those strings cannot be changed, ordered indexing where characters are accessed by position and support for various operations such as concatenation, slicing and iteration.

Strings are fundamental in Python for tasks such as text processing, input/output operations and data representation in applications offering a versatile toolset with built-in methods for efficient manipulation and formatting of textual information.

Creating Strings

Each type of string creation method i.e. ', ", """ has its own use case depending on whether we need to include quotes within the string, handle multi-line text or other specific formatting requirements in our Python code.

Following is the example of creating the string with the help od three types of quotes ', ", """ −

# Single line string
single_quoted_string = 'Hello, Welcome to Tutorialspoint'

# Double quoted string
double_quoted_string = "Python Programming"

# Triple quoted string for multi-line strings
multi_line_string = """This is a 
multi-line 
string"""

print(single_quoted_string)
print(double_quoted_string)
print(multi_line_string)

On executing the above code we will get the following output −

Hello, Welcome to Tutorialspoint
Python Programming
This is a
multi-line
string

Understanding String Immutability in Python

With the help of following example we are going to understand the immutability of the strings in python.

# Example demonstrating string immutability
my_string = "Hello"

# Attempting to modify a string will create a new string instead of modifying the original
modified_string = my_string + " Learners"
print(modified_string)  # Output: Hello Learners

# Original string remains unchanged
print(my_string)  # Output: Hello

# Trying to modify the string directly will raise an error
try:
   my_string[0] = 'h'  # TypeError: 'str' object does not support item assignment
except TypeError as e:
   print(f"Error: {e}")

On executing the above code we will get the following output −

Hello Learners
Hello
Error: 'str' object does not support item assignment

Frozen Sets

A frozen set in Python is an immutable version of a set. Once created its elements cannot be changed, added or removed. Frozen sets are particularly useful in situations where we need a set that remains constant throughout the execution of a program especially when we want to use it as a key in a dictionary or as an element in another set.

Creating Frozen Sets

We can create a frozen set using the frozenset() constructor by passing an iterable such as a list or another set as an argument. Following is the example of creating the Frozen set −

# Creating a frozen set
fset = frozenset([1, 2, 3, 4])

# Printing the frozen set
print(fset)  

On executing the above code we will get the following output −

frozenset({1, 2, 3, 4})

Understanding Frozen Sets Immutability in Python

Here's an example shows how frozensets being immutable and do not allow modifications after creation.

# Creating a frozenset
frozen_set = frozenset([1, 2, 3, 4])

# Attempting to add an element to the frozenset will raise an error
try:
   frozen_set.add(5)
except AttributeError as e:
   print(f"Error: {e}")

# Attempting to remove an element from the frozenset will also raise an error
try:
   frozen_set.remove(2)
except AttributeError as e:
   print(f"Error: {e}")

# The original frozenset remains unchanged
print("Original frozenset:", frozen_set)  

On executing the above code we will get the following output −

Error: 'frozenset' object has no attribute 'add'
Error: 'frozenset' object has no attribute 'remove'
Original frozenset: frozenset({1, 2, 3, 4})

Named Tuples

A Named tuple in Python is a lightweight data structure available in the collections module that behaves same as a tuple but allows us to access its elements using named attributes as well as indices. It combines the advantages of tuples such as immutable, memory-efficient with the ability to refer to elements by name, enhancing readability and maintainability of code.

Creating Named Tuples

we can define a named tuple using the namedtuple() factory function from the collections module. It takes two arguments such as a name for the named tuple type and a sequence i.e. string of field names or iterable of strings which specifies the names of its fields.

from collections import namedtuple

# Define a named tuple type 'Point' with fields 'x' and 'y'
Point = namedtuple('Point', ['x', 'y'])

# Create an instance of Point
p1 = Point(1, 2)

# Access elements by index (like a tuple)
print(p1[0])  

# Access elements by name
print(p1.x)   
print(p1.y)   

On executing the above code we will get the following output −

1
1
2

Understanding Named Tuples Immutability in Python

The Named tuples in Python are provided by the collections.namedtuple factory functions are indeed immutable. They behave similarly to regular tuples but have named fields by making them more readable and self-documenting.

from collections import namedtuple

# Define a named tuple called Point with fields 'x' and 'y'
Point = namedtuple('Point', ['x', 'y'])

# Create an instance of Point
p = Point(x=1, y=2)
print(p)  
# Attempt to modify the named tuple
# This will raise an AttributeError since named tuples are immutable
try:
   p.x = 10
except AttributeError as e:
   print(f"Error occurred: {e}")  

# Accessing elements in a named tuple is similar to accessing elements in a regular tuple
print(p.x) 
print(p.y) 

On executing the above code we will get the following output −

Point(x=1, y=2)
Error occurred: can't set attribute
1
2