Get unique values from a list in Python

A list in Python is a collection of items placed within [] which may contain duplicate values. In this article, we will explore different methods to extract only the unique values from a list while preserving or modifying the original order.

Using append() with Manual Checking

This approach creates a new empty list and appends elements only if they don't already exist. We use a for loop with not in condition to check for duplicates ?

Example

def get_unique_values(original_list):
    # Initialize an empty list
    unique_list = []
    
    # Check for elements
    for item in original_list:
        # Check if exists in unique_list
        if item not in unique_list:
            unique_list.append(item)
    
    return unique_list

days_list = ['Mon', 'Tue', 'Mon', 'Wed', 40, 40]
result = get_unique_values(days_list)
print("Original list:", days_list)
print("Unique values:", result)

Original list: ['Mon', 'Tue', 'Mon', 'Wed', 40, 40]
Unique values: ['Mon', 'Tue', 'Wed', 40]

Using set() Conversion

A set only contains unique values. This approach converts the list to a set (removing duplicates) and then back to a list. Note that this method doesn't preserve the original order ?

Example

days_list = ['Mon', 'Tue', 'Mon', 'Wed', 40, 40]
unique_set = set(days_list)
unique_list = list(unique_set)

print("Original list:", days_list)
print("Unique values:", unique_list)

Original list: ['Mon', 'Tue', 'Mon', 'Wed', 40, 40]
Unique values: [40, 'Wed', 'Tue', 'Mon']

Using NumPy unique()

NumPy provides a unique() function that returns sorted unique elements. This method converts all elements to strings and sorts them alphabetically ?

Example

import numpy as np

days_list = ['Mon', 'Tue', 'Mon', 'Wed', 40, 40]
unique_values = np.unique(days_list)

print("Original list:", days_list)
print("Unique values:", unique_values)
print("Type:", type(unique_values))

Original list: ['Mon', 'Tue', 'Mon', 'Wed', 40, 40]
Unique values: ['40' 'Mon' 'Tue' 'Wed']
Type: <class 'numpy.ndarray'>

Using dict.fromkeys() (Order Preserving)

This method preserves the original order while removing duplicates. Python dictionaries maintain insertion order since Python 3.7 ?

Example

days_list = ['Mon', 'Tue', 'Mon', 'Wed', 40, 40]
unique_list = list(dict.fromkeys(days_list))

print("Original list:", days_list)
print("Unique values (order preserved):", unique_list)

Original list: ['Mon', 'Tue', 'Mon', 'Wed', 40, 40]
Unique values (order preserved): ['Mon', 'Tue', 'Wed', 40]

Comparison

Conclusion

Use dict.fromkeys() for fast, order-preserving unique extraction. Use set() when order doesn't matter. Use np.unique() for numerical data that needs sorting.