The standard and expected behavior is that pasting *without* a selection inserts, pasting *with* a selection replaces.  Pasting with key, Edit menu, or context menu should be the same.  This is what I see and what I *think* you are describing.  If so, please close.  If not, please specify OS and version, how you installed Python, and more concretely what you did and whether line numbers or code context were present.