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

As the other answers have explained, the reason you're getting a list of None values is because list.append returns None, and you're calling it in a list comprehension. That means you're building a list full of None values along side your list of unique values.

I would like to suggest that you ditch the list comprehension. Because you need to access outside state (the list of unique values seen so far), a comprehension can't easily do what you want. A regular for loop is much more appropriate:

def remove_duplicates(lst):
    unique_list = []
    for item in lst:
        if item not in unique_list:
            unique_list.append(item)
    return unique_list

A more Pythonic approach however would be to use a set to handle the unique items, and to make your function a generator:

def remove_duplicates(lst):
    uniques = set()
    for item in lst:
        if item not in unique_list:
            yield item
            uniques.add(item)

The itertools.ifilterfase function from the standard library can help improve this even further, as shown in the recipe in the docs (you'll have to scroll down a little to find the specific recipe):

def unique_everseen(iterable, key=None):
    "List unique elements, preserving order. Remember all elements ever seen."
    # unique_everseen('AAAABBBCCDAABBB') --> A B C D
    # unique_everseen('ABBCcAD', str.lower) --> A B C D
    seen = set()
    seen_add = seen.add
    if key is None:
        for element in filterfalse(seen.__contains__, iterable):
            seen_add(element)
            yield element
    else:
        for element in iterable:
            k = key(element)
            if k not in seen:
                seen_add(k)
                yield element