l = [1, 2, 3]
s = set([2, 3, 4])
l += s
print(l) # Prints [1, 2, 3, 2, 3, 4]. Why?!
l = l +s # Raises TypeError (as expected)
What's going on? I would expect the attempt of adding a set to a list result in a TypeError. Why the difference between += and +?
