Extract of Modules/_tracemalloc.c:

static void
set_reentrant(int reentrant)
{
    assert(reentrant == 0 || reentrant == 1);
    if (reentrant) {
        assert(PyThread_get_key_value(tracemalloc_reentrant_key) == NULL);
        PyThread_set_key_value(tracemalloc_reentrant_key,
                               REENTRANT);
    }
    else {
        /* FIXME: PyThread_set_key_value() cannot be used to set the flag
           to zero, because it does nothing if the variable has already
           a value set. */
        PyThread_delete_key_value(tracemalloc_reentrant_key);
    }
}