Question

What is the most efficient way to determine if exactly two elements in a list are the same? For example:

>>> has1dup(["one", "one", "two"])
True
>>> has1dup(["one", "two", "three"])
False
>>> has1dup(["one", "one", "one"])
False

I have successfully done this using if/else statements. However, if the list were larger, the task of writing out each possibility for a pair would become very difficult and time consuming. Is there a faster/simpler way to accomplish this?

Here is what I have tried:

def has1dup(lst):
    if lst[0] == lst[1] and lst[1] != lst[2]:
        return True
    elif lst[1] == lst[2] and lst[2] != lst[0]:
        return True
    elif lst[0] == lst[2] and lst[2] != lst[1]:
        return True
    else:
        return False

Answer 1

You can see how many unique values are there with a set. If there is one less item in the set than in the list, one was a duplicate:

def has1dup(lst):
    return len(lst)-1 == len(set(lst))