What are the argument of IndexFlatL2.add() ?

[vittorio-perera]

In the python example code vectors are added to the index using:
index.add(xb)
on the other hand the C++ example uses:
index.add(nb, xb);
Checking the (python) code it looks like the signature for add is def add(self, n, x) but no default value is passed. What should x be? Asking as not passing x works but it will raise a pylint error