Speed? Not sure if this is actually what the OP was thinking of, but pass-by-value usually implies copying, while pass by reference just involves handing around

Speed? Not sure if this is actually what the OP was thinking of, but pass-by-value usually implies copying, while pass by reference just involves handing around a pointer. ie, if you had an enormous immutable value (in Python, a tuple with lots of entries), then in a hypothetical Python implementation which did call-by-value, it might be significantly slower than call-by-reference, on account of having to copy the entire tuple. But this falls squarely into 'implementation detail' - if you're so inclined, you could implement either CBV or CBR in hundreds of other ways. Certainly from the perspective of time-independent program behaviour you shouldn't be able to tell the difference.