On Tue, 19 May 2015 06:50 pm, Chris Angelico wrote: > On Tue, May 19, 2015 at 6:39 PM, Marko Rauhamaa <ma...@pacujo.net> wrote: >> For example, you could explain Python's object references as pointers >> (memory addresses) if you considered that helpful. (That's how Lisp >> textbooks often explain cons cells.) > > Sorta-kinda-maybe, but if a C programmer's idea of pointers is invoked > to explain Python's object references, the differences will start to > be problematic: > > 1) Pointer arithmetic simply doesn't exist in Python. Arrays/lists are > not just pointers to their first elements, and subscripting is most > definitely NOT "add to pointer and dereference". > 2) In fact, dereferencing as a whole isn't really a 'thing' either. At > best, it happens automatically. > 3) References actually mean something. Copying a pointer doesn't. > Whether the Python you're using is refcounted (CPython) or > mark-and-sweep (uPy, I think) or some other model, an additional > reference to the same object will prevent it from being disposed of, > which isn't the case in C. > 4) A pointer is itself a value. You can pass a > pointer-to-local-variable to another function and have that function > change a local variable. > 5) Furthermore, since a pointer is a value, you can have pointers to > pointers, etc. Doesn't make any sense in Python.
Chris, that is one of the best explanations for why "references equals pointers" is *not* a good explanation for Python's behaviour. I may have to steal it :-) -- Steven -- https://mail.python.org/mailman/listinfo/python-list
