Python: &= operator

Question

When I try to or/and two sets using &= and |= operator, I got some weird result.

s1 = {1,2,3}
s2 = {2,3,4}
tmp = s1
tmp &= s2 

As expected, tmp will be {2,3}, but I don't know why s1 also changed it value to {2,3}.

However, if I do:

tmp = tmp & s2

Then, s1 will be unchanged! Can anyone explain for me what happens underneath &= operator?

Answer 1

&= (set.__iadd__) for set is implemented differently with & (set.__add).

set &= ... is implemented using set.intersection_update which update the set in-place.

---

Relevant CPython code (Object/setobject.c):

set_iand(PySetObject *so, PyObject *other)
{
    PyObject *result;

    if (!PyAnySet_Check(other))
        Py_RETURN_NOTIMPLEMENTED;
    result = set_intersection_update(so, other); // <----
    if (result == NULL)
        return NULL;
    Py_DECREF(result);
    Py_INCREF(so);
    return (PyObject *)so;
}

Answer 2

It's called intersection_update. return set s keeping only elements also found in t. As you see in this picture;

You are re-building first set with intersection.