[enjo]

You just described why I believe pythons 'implicit is always better than explicit' is the most important language design fundamental ever conceived of.

The problem with the C++ approach is that it's default (pass by copy-constructor really) is rather implicit. Your doing a potentially a ton of work with a very innocent function call, with possible side-effects. At least the 'pass a reference by value' behaves the same way EVERY time.

For example, in C++:

myobj a; somefunction(a);

That copy constructor might trigger a database hit to create a new object if myobj was some sort of ORM. It might call 23 other constructors to fully complete it's deep copy. Who knows, as it all happens rather implicitly. In python, that would look like:

a = myobj() somefunction(a)

def somefunction(p): p = copy.deepcopy(p)

In that version I'm now implicitly providing that copy. In your multi-threaded example this is still better, as I get the best of both worlds. Ya, I might lose a bit of convenience... but that explictness is worth the trade-off IMHO.