How to remove duplicates from a list in Python
Jul 20, 2025 am 01:49 AMThere are three common methods for deduplication in Python. 1. Use set deduplication: It is suitable for situations where you don’t care about the order, and is implemented through list(set(my_list)). The advantage is that it is simple and fast, and the disadvantage is to disrupt the order; 2. Manually judge deduplication: By traversing the original list and determining whether the elements already exist in the new list, retaining the elements that appear for the first time, suitable for scenarios that need to maintain the order; 3. dict.fromkeys() deduplication: supported by Python 3.7, implemented through list(dict.fromkeys(my_list)), which maintains both the order and the writing method is concise. It is recommended to use modern Python. Notes include first converting the structure when dealing with non-hashable elements. It is recommended to use set records to improve efficiency in large data sets. Choosing the appropriate method depends on specific needs.
Deduplication is a common requirement when processing data, especially when processing lists in Python. The easiest and most effective method is to use sets, but how to operate depends on your actual needs.

Use set
to deduplication (for cases where you don't care about the order)
Sets are an unordered and non-repetitive data structure in Python. If your list does not need to remain in the original order, you can directly convert it to set and then return to list:
my_list = [1, 2, 2, 3, 4, 4, 5] unique_list = list(set(my_list))
This is simple and fast, but the disadvantage is that the order will be disrupted . So if your program depends on the order, it cannot be used like this.

Methods for deduplication keeping order
If the order is important to you, you can use an empty list to manually judge:
my_list = [1, 2, 2, 3, 4, 4, 5] unique_list = [] for item in my_list: if item not in unique_list: unique_list.append(item)
This code will retain the element position that appears for the first time , and the repeated subsequent items will be ignored. Although the writing method is a little bit verbose, the logic is clear and suitable for most scenarios.

Use dict.fromkeys()
to deduplicate (Python 3.7)
Starting in Python 3.7, dictionaries maintain insertion order by default. You can use this feature to deduplicate:
my_list = [1, 2, 2, 3, 4, 4, 5] unique_list = list(dict.fromkeys(my_list))
This method is both orderly and concisely written and is recommended in modern Python.
Notes and details
- If the list contains non-hashable elements (such as a list nested in the list), using set or dict directly will report an error, and the internal structure needs to be converted or processed first.
- For large data sets, cyclically determine whether it is less efficient in the list. You can consider recording the elements that have appeared with a collection.
- In most cases,
dict.fromkeys()
is recommended unless you are using an older version of Python.
Basically that's it. The deduplication method is not complicated, but it is important to choose the right method according to the specific scenario.
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