Python : Copying Sets

In Python, sets are unordered collections of unique elements. Sets are mutable, meaning you can add, remove, or modify their elements after creation.

When working with sets, you may need to create a copy of a set for various reasons, such as preserving the original set while working on a modified version.

Python provides several ways to copy sets, including shallow copies using the copy() method, the set() function, and more.

This tutorial will guide you through the different methods of copying sets, with practical examples.

1. Using the copy() Method

The copy() method creates a shallow copy of a set. This method is the most straightforward way to copy a set, creating a new set that contains all the elements of the original set. Modifying the new set will not affect the original set.

Example: Copying a Set Using copy()

# Example: Using copy() to copy a set
original_set = {1, 2, 3, 4}

# Create a copy of the set
copied_set = original_set.copy()

# Modify the copied set
copied_set.add(5)

# Print both sets
print("Original set:", original_set)  # Output: {1, 2, 3, 4}
print("Copied set:", copied_set)      # Output: {1, 2, 3, 4, 5}

In this example:

original_set.copy() creates a shallow copy of the set. Changes made to copied_set do not affect original_set.

2. Using the set() Constructor

Another way to copy a set is by using the set() constructor. This method works similarly to copy() by creating a new set containing all the elements of the original set.

Example: Copying a Set Using set()

# Example: Using set() to copy a set
original_set = {10, 20, 30}

# Create a copy of the set using the set() constructor
copied_set = set(original_set)

# Modify the copied set
copied_set.add(40)

# Print both sets
print("Original set:", original_set)  # Output: {10, 20, 30}
print("Copied set:", copied_set)      # Output: {10, 20, 30, 40}

In this example:

set(original_set) creates a new set with the same elements as original_set. Changes to copied_set do not affect original_set.

3. Shallow Copy vs Deep Copy

In Python, the copy() method creates a shallow copy, meaning that only the outer structure (the set) is copied, and any references to objects inside the set are still pointing to the original objects.

This is important when the set contains mutable objects, like lists.

Example: Shallow Copy of a Set Containing Mutable Objects

# Example: Shallow copy of a set containing a list
original_set = {1, 2, [3, 4]}

# Copy the set using copy()
copied_set = original_set.copy()

# Modify the list inside the copied set
copied_set.remove([3, 4])
copied_set.add([5, 6])

# Print both sets
print("Original set:", original_set)  # Output: {1, 2, [3, 4]}
print("Copied set:", copied_set)      # Output: {1, 2, [5, 6]}

In this example:

copied_set is a shallow copy of original_set, so the list [3, 4] is not deeply copied. Modifications to mutable objects (like the list) will reflect in both sets.

4. Copying a Set in a Loop

You can also copy the elements of one set into another set using a loop. This approach may not be as efficient as using the copy() method or the set() constructor, but it's an alternative way to understand how copying works.

Example: Copying a Set Using a Loop

# Example: Copying a set using a loop
original_set = {1, 2, 3, 4}
copied_set = set()

# Manually copy elements from original_set to copied_set
for item in original_set:
    copied_set.add(item)

# Modify the copied set
copied_set.add(5)

# Print both sets
print("Original set:", original_set)  # Output: {1, 2, 3, 4}
print("Copied set:", copied_set)      # Output: {1, 2, 3, 4, 5}

In this example:

The loop iterates through each element of original_set and adds it to copied_set. This is a manual approach to copying.

5. Copying Immutable Sets: Frozen Sets

In Python, frozen sets are immutable versions of sets. You cannot modify a frozen set after its creation, so there is no copy() method available.

However, you can create a new frozen set using the frozenset() function, which acts like a “copy” in this case.

Example: Copying a Frozen Set

# Example: Copying a frozen set
original_frozenset = frozenset([1, 2, 3])

# "Copy" the frozen set by creating a new frozenset
copied_frozenset = frozenset(original_frozenset)

# Print both frozen sets
print("Original frozen set:", original_frozenset)  # Output: frozenset({1, 2, 3})
print("Copied frozen set:", copied_frozenset)      # Output: frozenset({1, 2, 3})

In this example:

frozenset(original_frozenset) creates a new frozen set with the same elements as the original frozen set.

6. Comparing Copies

After copying a set, you may want to check if the new set is equal to the original set and if they are the same object in memory.

Example: Checking Equality and Identity

# Example: Comparing sets after copying
original_set = {1, 2, 3}
copied_set = original_set.copy()

# Check if the sets are equal (same elements)
print(original_set == copied_set)  # Output: True

# Check if the sets are the same object in memory
print(original_set is copied_set)  # Output: False

In this example:

original_set == copied_set checks if both sets have the same elements (True).
original_set is copied_set checks if they refer to the same object in memory (False).

7. Deep Copy of a Set Containing Mutable Elements

When a set contains mutable elements (like lists), and you want to copy both the set and its contents, you need to perform a deep copy using the copy module’s deepcopy() function.

Example: Deep Copy of a Set with Mutable Elements

import copy

# Example: Deep copy of a set with mutable elements
original_set = {1, 2, [3, 4]}

# Perform a deep copy
copied_set = copy.deepcopy(original_set)

# Modify the list inside the copied set
copied_set.remove([3, 4])
copied_set.add([5, 6])

# Print both sets
print("Original set:", original_set)  # Output: {1, 2, [3, 4]}
print("Copied set:", copied_set)      # Output: {1, 2, [5, 6]}

In this example:

The deepcopy() function creates a deep copy of the set, including a copy of the list inside it. Changes to the copied set do not affect the original set.

Summary

The copy() method is the simplest way to create a shallow copy of a set.
You can also use the set() constructor to copy a set, which works similarly to copy().
Shallow copies copy the outer set structure but retain references to mutable objects inside the set.
For frozen sets, you can “copy” them by creating a new frozen set using frozenset().
Use the deepcopy() function from the copy module if your set contains mutable elements and you need to create independent copies of both the set and its contents.
You can compare the equality of sets and their identity in memory using == and is.

By understanding how to copy sets in Python, you can manipulate and preserve data more effectively in your programs while ensuring that you maintain control over the original and copied sets.