[python] Combining two lists and removing duplicates, without removing duplicates in original list

I have two lists that i need to combine where the second list has any duplicates of the first list ignored. .. A bit hard to explain, so let me show an example of what the code looks like, and what i want as a result.

first_list = [1, 2, 2, 5]

second_list = [2, 5, 7, 9]

# The result of combining the two lists should result in this list:
resulting_list = [1, 2, 2, 5, 7, 9]

You'll notice that the result has the first list, including its two "2" values, but the fact that second_list also has an additional 2 and 5 value is not added to the first list.

Normally for something like this i would use sets, but a set on first_list would purge the duplicate values it already has. So i'm simply wondering what the best/fastest way to achieve this desired combination.

Thanks.

resulting_list = first_list + [i for i in second_list if i not in first_list]

Simplest to me is:

first_list = [1, 2, 2, 5]
second_list = [2, 5, 7, 9]

merged_list = list(set(first_list+second_list))
print(merged_list)

#prints [1, 2, 5, 7, 9]

resulting_list = list(first_list)
resulting_list.extend(x for x in second_list if x not in resulting_list)

first_list = [1, 2, 2, 5]
second_list = [2, 5, 7, 9]

print( set( first_list + second_list ) )

You can also combine RichieHindle's and Ned Batchelder's responses for an average-case O(m+n) algorithm that preserves order:

first_list = [1, 2, 2, 5]
second_list = [2, 5, 7, 9]

fs = set(first_list)
resulting_list = first_list + [x for x in second_list if x not in fs]

assert(resulting_list == [1, 2, 2, 5, 7, 9])

Note that x in s has a worst-case complexity of O(m), so the worst-case complexity of this code is still O(m*n).

This might help

def union(a,b):
    for e in b:
        if e not in a:
            a.append(e)

The union function merges the second list into first, with out duplicating an element of a, if it's already in a. Similar to set union operator. This function does not change b. If a=[1,2,3] b=[2,3,4]. After union(a,b) makes a=[1,2,3,4] and b=[2,3,4]

Based on the recipe :

resulting_list = list(set().union(first_list, second_list))

You can use sets:

first_list = [1, 2, 2, 5]
second_list = [2, 5, 7, 9]

resultList= list(set(first_list) | set(second_list))

print(resultList)
# Results in : resultList = [1,2,5,7,9]

You can bring this down to one single line of code if you use numpy:

a = [1,2,3,4,5,6,7]
b = [2,4,7,8,9,10,11,12]

sorted(np.unique(a+b))

>>> [1,2,3,4,5,6,7,8,9,10,11,12]

    first_list = [1, 2, 2, 5]
    second_list = [2, 5, 7, 9]

    newList=[]
    for i in first_list:
        newList.append(i)
    for z in second_list:
        if z not in newList:
            newList.append(z)
    newList.sort()
    print newList

[1, 2, 2, 5, 7, 9]