On Wednesday 10 April 2002 01:53 pm, Geoffrey Talvola wrote:
> I would propose using "hasattr" to check for the existence of awake,
> respond, and sleep, and removing the assert statement. The
> RuntimeError that results could have a much better error message than
> the assert would give us, anyhow.
>
> But I'm curious on Chuck's take on removing the assert statement. I
> somehow have a feeling that he has an opinion on this, maybe because
> it was discussed before?
>
> - Geoff
Yes, this was discussed and decided before. Here are the points:
- By requiring subclasses of Servlet, we can put methods in Servlet
that other parts of WebKit (like the AppServer and Application) can
make use of.
Otherwise, if we decide to add some method like "serverSidePath" in a
version 0.4 then we basically break every WebKit application that
wasn't subclassing Servlet.
We would either get breakage every time we added something (when there
is often a reasonable default implementation), or we could have to make
these methods into global functions (that would presumably check
hasattr() first to see if the servlet wanted to "override them").
There is a "myth" among the Python community that requiring inheritance
is _always_ bad. It is bad if you only rely on one or two methods and
don't plan on beefing up the de facto base class. It is also bad with
builtin types in Py <= 2.2 because they couldn't be subclassed.
- There are probably more required methods than you might think. I
doubt that it's only awake, respond and sleep.
- Since you are already using WebKit, inheriting from Servlet (or a
subclass is extremely easy):
from WebKit.HTTPServlet import HTTPServlet
class Foo(HTTPServlet):
pass
- Python provides multiple inheritance so you're not locked into just
inheriting from WebKit.
- Invoking hasattr() multiple times will be more expensive than one
isinstance().
- Checking isinstance() guards against the "duplicate module" bug we
experienced earlier.
It sounds like the assert should be changed to an "if: raise" to be
more clear and avoid being "optimized out".
-Chuck
>
> > -----Original Message-----
> > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]
> > Sent: Monday, April 08, 2002 3:31 PM
> > To: [EMAIL PROTECTED]
> > Subject: Re: [Webware-discuss] Allowing delegation in servlets
> >
> > On Mon, Apr 08, 2002 at 02:43:37PM -0400, Geoffrey Talvola wrote:
> > > > My first attempt failed because line 183 of ServletFactory is:
> > > >
> > > > assert issubclass(theClass, Servlet)
> > > >
> > > > Anyway, I'd like to argue for the removal of the assertion in
> > > > ServletFactory, since it is denying use of a very powerful,
> > > > and arguably
> > > > better technique than inheritance. In any case, assertions
> > > > can be optimized
> > > > away in .pyo files, so that the presence of the assertion
> > > > statement is of
> > > > questionable value...
> > >
> > > I think the assertion is there to help out the Webware user
> >
> > who forgets to
> >
> > > have their servlet inherit from Servlet. I assume that
> >
> > mistake happens
> >
> > > relatively frequently. I know I've done it once or twice.
> >
> > Without the
> >
> > > assertion (which pinpoints the problem pretty clearly),
> >
> > presumably you would
> >
> > > wind up with a traceback somewhere else in the guts of
> >
> > Appliction.py that
> >
> > > would be less obvious to track down, especially for a casual
> > > user.
> > >
> > > Now you've identified a legitimate use for a servlet not
> >
> > derived from
> >
> > > Servlet. So you want to remove the assertion, but that
> >
> > would hurt casual
> >
> > > Webware users hwo might forget the base class. So maybe
> >
> > there should be an
> >
> > > option "AllowServletsNotDerivedFromServlet" that defaults
> >
> > to 0, but you can
> >
> > > just flip the switch and it skips the assertion? I would
> >
> > imagine this to be
> >
> > > documented as "for experts".
> >
> > If a servlet must always subclass Servlet, or at least we pretend
> > to require that for the default case, the exception should be
> > changed from
> > AssertionError to something else. AssertionError mean "I" made an
> > error. Other exceptions mean "you" made an error.
> >
> > But perhaps we need to rethink what are the requirements for
> > servlets. At minimum, awake/respond/sleep methods. This sounds
> > like an interface.
> > Since Python doesn't have interfaces (yet), we've been using
> > "must be a
> > subclass of Servlet" as a substitute.
> >
> > The simplest way to avoid the issubclass test and yet not introduce
> > an esoteric option like AllowServletsNotDerivedFromServlet would be
> > to test explicitly for the required features:
> >
> > if not hasattr(theClass, 'respond'): raise RuntimeError(...)
> >
> > try:
> > theClass.respond
> > except AttributeError:
> > raise RuntimeError(...)
> >
> > if not hasattr(theClass, 'respond') or \
> > type(theClass.respond) != types.MethodType:
> > raise RuntimeError(...)
> >
> > requiredMethods = 'awake', 'respond', 'sleep'
> > for methMame in requiredMethods:
> > try:
> > meth = getattr(theClass, methName)
> > if type(meth) != types.MethodType:
> > raise RuntimeError("not a method...")
> > except AttributeError:
> > raise RuntimeError("method doesn't exist...")
> >
> >
> > --
> > -Mike (Iron) Orr, [EMAIL PROTECTED] (if mail problems: [EMAIL PROTECTED])
> > http://iron.cx/ English * Esperanto * Russkiy *
> > Deutsch * Espan~ol
> >
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