Derek Martin wrote: > On Tue, Dec 30, 2008 at 02:21:29PM +0000, John O'Hagan wrote: [...] > What the Python community often overlooks, when this discussion again > rears its ugly head (as it seems to every other hour or so), is that > its assignment model is BIZARRE, as in it's conceptually different > from virtually all other languages substantially taught in > undergraduate computer science programs. And for that matter, it's > pretty unintuitive generally. > I'd definitely argue against bizarre. It's actually very easy to understand, and Python is by no means the only language to have used it.
> That is, in what I'll call "normal" computer languages, a variable > name is thought of as the address of a bin where some data is stored, > and the name is inexorably tied to that bin. You can change what's in > the bin, but the name you gave the bin always points to the same bin. > This tends to be conceptually true even if it might technically not be > true in a given implementation of a language. > I'd argue that this approach is out of date and overly-restrictive, requiring data copying not required by the Python data model and running the risk, in the absence of such copying, of returning invalid references (the C++ "dangling pointer" issue being the best-known manifestation of such problems). > Python is very different from this. Names are not addresses of bins; > they are instead simply ephemeral labels which are given to bins, > where the bin is a Python object which contains specific data at the > time of assignment. A second assignment of that name doesn't change > what's in the original bin; it actually (probably) first creates a new > bin, then removes the name from the original bin and assigns it to > the new one. Intuitively, it's a bit like saying your kitchen table > is no longer a kitchen table, and now the thing where you wash your > dishes is a kitchen table. It doesn't really make a lot of sense > (whether or not it's so for good reason), and it makes describing the > assignment model necessarily convoluted, whereas the "named bins" > model from the majority of other languages people are likely to have > been exposed to is simple and sensible. > I'd instead say that Python uses ephemeral names for long-lived objects, where other languages use the addresses of ephemeral objects. Your ideas of "simple" and "sensible" are being conditioned by your experience. > It's small wonder that neophytes try to cram Python behaviors into > terms and computing concepts they already understand from learning > other languages, and that they fail to do so. What's mystifying is > that when Pythonistas reply to their messages, they universally seem > confused at how this could possibly happen, and often enough actually > seem offended (or at least offensive) when it inevitably does happen... > Generally speaking we try not to be offensive first on this list. regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
