> So s.append(x) just copies the pointer, not the actual value.

Yes, that's a reasonable way to think about it (though the term 'reference' seems to more popular than 'pointer' in this context).  It matches the implementation, too: internally, a list is represented as a resizable array of pointers to objects.  (For CPython, anyway;  alternative implementations of Python might differ.)

Implicit object copies are rare in Python.

comp.lang.python is a good place to ask questions about Python's object model.