Arguably, it's a matter of isolation from 'irrelevant' changes. In a function, one wishes to keep the ability to redefine their local variables (perhaps, to imp

Arguably, it's a matter of isolation from 'irrelevant' changes. In a function, one wishes to keep the ability to redefine their local variables (perhaps, to implement a faster/smaller-space algorithm) without otherwise affecting the overall program -- when they call other code, that other code should avoid accessing the caller's variables, lest the caller be stuck with precisely those variables and that implementation. If this isolation exists, I have more freedom to vary the implementation. Likewise, private members of a class are implementation details I may wish to change later -- but I would like some assurance that other components will be isolated from this change, so as not to break. Edit-add: oh, I of course forgot to mention -- for those not familiar, python permits both :-)