How to Convert a List into Ordered Set in Python?

The task is to turn a list into an ordered set, meaning we want to remove any duplicate items but keep the order in which they first appear. In this article, we will look at different ways to do this in Python.

Using OrderedDict.fromkeys()

OrderedDict.fromkeys() method removes duplicates from a list and keeps the original order of the elements. It works by using an OrderedDict, which remembers the order of items as they are added.

Example:

from collections import OrderedDict

# list
a = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3]

# Remove duplicates while preserving order
res = list(OrderedDict.fromkeys(a))

# Print the result
print(res)

[3, 1, 4, 5, 9, 2, 6]

Using dict.fromkeys()

dict.fromkeys() method removes duplicates from a list while preserving the order of elements. Since dictionaries in Python 3.7+ maintain insertion order.

Example:

# list
a = [1, 2, 3, 2, 1, 4, 5]

# Remove duplicates while preserving order
res = list(dict.fromkeys(a))
print(res)

[1, 2, 3, 4, 5]

Using for loop

for loop iterate through the list, tracking seen elements with a set. It removes duplicates while preserving the order of the original list.

Example:

# list
a = [1, 2, 3, 2, 1, 4, 5]

# Initialize an empty list to store the result 
res = []  
# Initialize set `s` to track seen elements
s = set()

# Iterate through `a`
for i in a :
    # If the item is not already in `s`
    if i not in s:
        # Add the item to the `res`
        res.append(i)
        # Mark item as seen by adding it to `s`
        s.add(i)
print(res)

[1, 2, 3, 4, 5]