Basic example
Input
unique_preserve([1, 2, 2, 3, 1])
Output
[1, 2, 3]
1 appears first, then 2, then 3. Subsequent duplicates are ignored.
Full lesson preview
Get unique elements preserving order
Return the unique elements from a list while keeping their first-seen order.
Python practice16 minLists & TuplesIntermediateLast updated March 17, 2026
Problem statement
Write a function unique_preserve(lst) that takes a list and returns a new list with only the first occurrence of each element, preserving the original order. Do not use sorting. The function should handle any hashable elements (numbers, strings, tuples) and must preserve the order in which distinct elements first appear.
Task
Learn how to remove duplicates from a list without changing the relative order of first occurrences.
Examples
Input format
A single list (e.g., [1, 2, 2, 3]) passed to unique_preserve.
Output format
A list containing the first occurrence of each distinct element, in original order.
Constraints
You may assume elements are hashable. Do not change the input list in-place (return a new list). Aim for O(n) time and O(n) extra space.
Samples
Sample 1
Input
unique_preserve([5, 5, 6, 7, 6])
Output
[5, 6, 7]
Keeps first 5, first 6, and first 7; ignores later duplicates.