On Tue, May 22, 2012 at 4:07 PM, Dan Goodman <dg.gm...@thesamovar.net> wrote: > On 22/05/2012 18:20, Nathaniel Smith wrote: >> I don't know of anything that the docs are lacking in particular. It's >> just that subclassing in general is basically a special form of >> monkey-patching: you have this ecosystem of cooperating methods, and >> then you're inserting some arbitrary changes in the middle of it. >> Making it all work in general requires that you carefully think >> through how all the different pieces of the ndarray API interact, and >> the ndarray API is very large and complicated. The __getslice__ thing >> is one example of this. For another: does your __getitem__ properly >> handle *all* the cases that regular ndarray.__getitem__ handles? (I'm >> not sure anyone actually knows what this complete list is, there are a >> lot of potential corner cases.) What happens if one of your objects is >> passed to third-party code that uses __getitem__? What happens if your >> array is accidentally stripped of its magic properties by passing >> through np.asarray() at the top of some function? Have you thought >> about how your special attributes are affected by, say, swapaxes? Have >> you applied your tweaks to item() and setitem()? >> >> I'm just guessing randomly here of course, since I have no idea what >> you've done. And I've subclassed ndarray myself at least three times, >> for reasons that seemed good enough at the time, so I'm not saying >> it's never doable. It's just that there are tons of these tiny little >> details, any one of which can trip you up, and that means that people >> tend to dive in and then discover the pitfalls later. > > I've also used subclasses of ndarray, and have stumbled across most (but > not all) of the problems you mentioned above. In my case, my code has > gradually evolved over a few years as I've become aware of each of these > problems. I think it would be useful to have an example of a completely > 'correctly' subclassed ndarray that handles all of these issues that > people could use as a template when they want to subclass ndarray. I > appreciate, though, that there's no-one who particularly wants to do > this! :) I'd offer my code as an example, but Nathaniel's comment above > shows that there's many things mine doesn't handle properly. > > Dan > > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion >
The text Nathaniel wrote is already very useful (certainly to me). It seems like this text could be put almost verbatim (maybe with some list items) in a subsection at the end of [1] titled "Caution" or "Other considerations". Thanks, Tom [1] http://docs.scipy.org/doc/numpy/user/basics.subclassing.html _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
