Andrew Koenig writes: > I started the thread because I wanted to call attention to an issue: Objects > sometimes have properties that have side effects when exploited, and it can > be useful to be able to test for the presence of such properties without > evoking the side effects.
Good point. I agree. I cannot list every possible property, but so far three have been mentioned. For callability and iterability, I think that hasattr(x, '__call__') and hasattr(x, '__iter__') are perfectly good spellings for the test. You say: > I felt uncomfortable about exposing the implementation that way ...but double-underscore methods are part of the language definition, not part of the implementation. If we were saying to check whether certain tp* fields in a C struct were populated, then I would object, but checking for double-underscore methods doesn't expose implementation. (It might, however, require the C implementation to set some extra fields just to simulate having a __iter__ when it really relies on tpIter. No big deal.) Now, as Greg has pointed out, hashability cannot be tested in this manner: > since object has __hash__ we end up concluding that everything is > hashable Fortunately, I (and apparently others) believe this limitation is OK because there is no need to test for hashability without evoking the side effects. In general, __hash__ should be "quick" (which may be a relative term) and return the same value each time it is called, so simply calling __hash__ to find whether it works ought to do nicely. --- Much earlier, Andrew had also asked me this: > Perhaps we are using "idempotent" to mean different things. I think of a > function f as being idempotent if f(x) == x for all x in dom(f). Apparently so. In mathematics, the term means a function where f(f(x)) = f(x) for all x. But in programming I have always understood it to mean a function which will return the same result no matter how many times it is invoked. In a quick glance at internet dictionaries I found surprisingly little support for this definition, but I still believe it is a fairly widely-used definition as I have heard it used by co-workers from more than one job. The one source I DID find was "http://en.wiktionary.org/wiki/idempotent";. -- Michael Chermside _______________________________________________ Python-3000 mailing list Python-3000@python.org http://mail.python.org/mailman/listinfo/python-3000 Unsubscribe: http://mail.python.org/mailman/options/python-3000/archive%40mail-archive.com
