On 2008-11-13 09:03, Peter Otten wrote: > jalanb3 wrote: > >> Evening all, >> >> And thank you for your valuable reading skills. >> >> The following pattern turned up in coding tonight. >> It works, but I'm suspicious, it just seems too easy. >> >> So any comments or constructive criticisms welcome ? >> >> *** Start doctest format *** >> >>>>> class Cacher: >> ... def __init__(self): >> ... self.value_in_database = 0 >> ... >> ... def very_slow_database_access(self): >> ... return self.value_in_database >> ... >> ... @property >> ... def show(self): >> ... result = self.very_slow_database_access() >> ... self.show = result >> ... return result >> ... > > The above only works because properties don't work with classic class. If > you make Cacher a newstyle class (have it inherit from object) you'll get > an AttributeError. Therefore I think it would be cleaner to either > > use __getattr__() or > > change the Cacher to something like > > class Cacher(object): > def __init__(self): > self._show = None > # ... > > @property > def show(self): > result = self._show > if result is None: > result = self._show = self.very_slow_database_access() > return result
You should also consider not using a property for this at all. Attribute access is supposed to be fast and, if at all, only cause AttributeErrors. Your self.very_slow_database_access() call can raise all sorts of errors and likely take much too long for what a casual user would expect from writing print (cache.show). It's much better to write this as method which then uses an instance attribute for caching the value read from the database. -- Marc-Andre Lemburg eGenix.com Professional Python Services directly from the Source (#1, Nov 13 2008) >>> Python/Zope Consulting and Support ... http://www.egenix.com/ >>> mxODBC.Zope.Database.Adapter ... http://zope.egenix.com/ >>> mxODBC, mxDateTime, mxTextTools ... http://python.egenix.com/ ________________________________________________________________________ :::: Try mxODBC.Zope.DA for Windows,Linux,Solaris,MacOSX for free ! :::: eGenix.com Software, Skills and Services GmbH Pastor-Loeh-Str.48 D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg Registered at Amtsgericht Duesseldorf: HRB 46611 -- http://mail.python.org/mailman/listinfo/python-list
