On Sun, 15 May 2005 14:31:21 -0400, "Terry Reedy" <[EMAIL PROTECTED]> wrote:
> >"Jordan Rastrick" <[EMAIL PROTECTED]> wrote in message >news:[EMAIL PROTECTED] > >> I've read arguments, here and elsewhere, to the effect that in Python >> isinstance should be avoided like the plague, except in a few very >> specific and narrow circumstances. > >Putting it like this is rather extreme. > >> Roughly speaking, due in part to >> Python's dynamic nature its better to concern yourself only with the >> interface an object provides, and not whether it happens to inherit >> from a given base class. > >To my mind, your example of using isinstance to select a better (such as >speedier) subalgorithm for a special case is not just fine, but good >programming. (Selecting a subalgorithm that works more robustly is also a >good reason for special casing.) It is an internal matter whose externally >visible effect is to improve performance. I agree, but I am also a little uncomfortable about such performance tuning, unless the assumptions it depends on are prominently documented or even validated with an assert or explicit warning. Otherwise the next version of the interpreter or a library module could change the optimal decision, and a bad optimization decision could be locked in for the new version. Maybe there should be another testable condition like __debug__ except for testing (e.g. __testing__ ;-) which could be used to introduce temporary alternative code (such as alternate optimization decisions) so that system tests could be used to validate locking in one decision or another for a new system version being tested. For trivial personally maintained code, a one-line version check with a reminder exception to re-visit the optimization or whatever decision (and revise the version check for next time) could cheaply prevent hidden lock-in of bad optimization etc. > >Using isinstance to unnecessarily narrow the domain is quite different. It >has the externally visible effect of degrading performance (to a nullity) >for arguments that the user might reasonably want to work. Agreed, but the key thing there is to define "unnecessarily" ;-) Regards, Bengt Richter -- http://mail.python.org/mailman/listinfo/python-list
