Python/import.c lines: 1078-1082:

	for (j = 0; j < nhooks; j++) {
		PyObject *hook = PyList_GetItem(path_hooks, j);
		if (hook == NULL)
			return NULL;
		importer = PyObject_CallFunctionObjArgs(hook, p, NULL);

The "hook" object is supposed to be a callable, not a string.  The code
for PyList_GetItem() does its own argument checking.