Ordered Sets in Python

Introduction

In Python, the built-in set data structure provides a way to store unique, unordered elements. However, many applications require a set that not only guarantees uniqueness but also maintains the order in which elements were inserted. This tutorial explores different approaches to creating ordered sets in Python, from leveraging existing data structures to using dedicated third-party libraries.

Why Ordered Sets?

Traditional sets in Python do not preserve insertion order. This can be problematic when the order of elements is significant for subsequent operations or analysis. Ordered sets are useful in scenarios such as:

• Data Processing Pipelines: Maintaining the sequence of items as they pass through stages.
• Duplicate Removal with Order Preservation: Removing duplicates from a list while retaining the original order.
• Implementing Specific Algorithms: Certain algorithms rely on the order of elements in a set-like structure.

Using Dictionaries as Ordered Sets

From Python 3.7 onwards (and as an implementation detail in CPython 3.6), standard dictionaries are guaranteed to preserve insertion order. This property can be exploited to create an ordered set by using a dictionary where keys represent the elements of the set, and values are simply ignored (often set to None).

Here’s how you can implement an ordered set using a dictionary:

def ordered_set_from_iterable(iterable):
    """Creates an ordered set from an iterable."""
    return dict.fromkeys(iterable)

# Example Usage
keywords = ['foo', 'bar', 'bar', 'foo', 'baz', 'foo']
unique_keywords = list(ordered_set_from_iterable(keywords))
print(unique_keywords)  # Output: ['foo', 'bar', 'baz']

In this approach, dict.fromkeys(iterable) creates a dictionary with the elements of iterable as keys and None as values. Since dictionary keys are unique, duplicates are automatically removed. Converting the dictionary keys back to a list (list(dict.fromkeys(iterable))) provides the ordered set.

This method is simple, efficient, and leverages a built-in data structure. It’s generally the preferred solution for modern Python versions.

Using collections.OrderedDict

Prior to Python 3.7, or when backward compatibility is crucial, collections.OrderedDict provides an explicit way to create an ordered dictionary. You can use it in a similar manner to the dictionary approach described above.

from collections import OrderedDict

def ordered_set_from_iterable_ordereddict(iterable):
    """Creates an ordered set from an iterable using OrderedDict."""
    return OrderedDict.fromkeys(iterable)

# Example Usage
keywords = ['foo', 'bar', 'bar', 'foo', 'baz', 'foo']
unique_keywords = list(ordered_set_from_iterable_ordereddict(keywords))
print(unique_keywords)  # Output: ['foo', 'bar', 'baz']

The logic is identical to the dictionary example, but using OrderedDict explicitly guarantees order preservation across all Python versions.

Third-Party Libraries

Several third-party libraries provide dedicated ordered set implementations:

ordered-set

The ordered-set package provides a simple, Python-based implementation of an ordered set. You can install it using pip install ordered-set.

from ordered_set import OrderedSet

# Example Usage
keywords = ['foo', 'bar', 'bar', 'foo', 'baz', 'foo']
unique_keywords = list(OrderedSet(keywords))
print(unique_keywords)  # Output: ['foo', 'bar', 'baz']

boltons.setutils.IndexedSet

The boltons library offers a more feature-rich IndexedSet which provides both order preservation and indexing capabilities. Install it with pip install boltons.

from boltons.setutils import IndexedSet

# Example Usage
keywords = ['foo', 'bar', 'bar', 'foo', 'baz', 'foo']
unique_keywords = list(IndexedSet(keywords))
print(unique_keywords) # Output: ['foo', 'bar', 'baz']

# Demonstrate indexing
print(unique_keywords[1]) # Output: bar

IndexedSet is particularly useful when you need to access elements by their position in the ordered set.

oset and other packages

Other packages like oset also exist, but their last updates may be dated. Consider the maintenance and community support when choosing a third-party library.

Choosing the Right Approach

The best approach depends on your specific needs:

• Python 3.7+: Using a standard dictionary with dict.fromkeys() is the simplest and most efficient solution.
• Backward Compatibility: Use collections.OrderedDict for guaranteed order preservation in older Python versions.
• Indexing Required: boltons.setutils.IndexedSet provides both order preservation and indexing functionality.
• Simple Implementation: The ordered-set package offers a lightweight and straightforward implementation.