Finding minimum excludant of a set

I am a total beginner in Python.
Can’t figure out why the following doesn’t work when I am trying to find the minimum excludant of the given set.

def mex(my_list):
    my_list = set(my_list)
    mex = 0
    while mex in my_list:
        mex += 1
    return mex

a = [1, 0, 2, 4]
print(mex(a))

>Solution :

Your return needs to be indented and you need a ":" after the while statement.

def mex(my_list):
    my_list = set(my_list)
    mex = 0
    while mex in my_list:
        mex += 1
    return mex

a = [1, 0, 2, 4]
print(mex(a))