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How to Remove Duplicates from a List in Python

Oct 25, 2020 1 min read

We want to remove duplicates from a list, or in other words, keep only unique values in our list.

duplicates = [0, 0, 0, 1, 1, 1, 2, 2, 2]

Using a for Loop

The brute force, inefficient way to remove duplicates is using a for loop, which might look something like this.

unique = []
for num in duplicates:
  if num not in unique:
    unique.append(num)
# [0, 1, 2]

Using set()

We can avoid this loop by simply converting the duplicates list into a set.

By definition, a set will only contain unique items.

We can then convert the set back into a list if needed.

unique = list(set(duplicates))
# [0, 1, 2]

The issue here is that a set is unordered, so the original list order is not guaranteed after the set-to-list conversion.

So, how can we preserve order?

Using dict.fromkeys() to Preserve Order

dict.fromkeys(list) will return a dictionary with the list items as keys. Dictionaries do not allow duplicate keys, so the returned dictionary will remove the duplicates for us while preserving order.

dict.fromkeys(duplicates)
# {0: None, 1: None, 2: None}

We can then convert back to a list.

unique = list(dict.fromkeys(duplicates))
# [0, 1, 2]