Why does removing duplicates from a list produce [None, None] output?

Question

I am new to Python and I\'m not able to understand why I am getting the results with None values.

#Remove duplicate items from a list
def remove         


        

Answer 1

Although using a set is the proper way, the problem with your code, as the comments indicated, is that you are not actually returning unique_list from your function, you are returning the result of the list comprehension.

def remove_duplicates(my_list):
    unique_list = []
    do = [unique_list.append(item) for item in my_list if item not in unique_list]
    return unique_list  # Actually return the list!

print remove_duplicates([1,1,2,2]) -> result [1, 2]

Here I simply made a throwaway variable do that is useless, it just "runs" the comprehension. Understand?

That comprehension is storing a value each time you call unique_list.append(item) ... and that value is the result of the append method, which is None! So do equals [None, None].

However, your unique_list is in fact being populated correctly, so we can return that and now your function works as expected.

Of course, this is not a normal use for a list comprehension and really weird.