RE: [Jython-users] Running auto-discovered methods

[brian z. <bz...@zi...>]

Neil,

What I was trying to say was some functions require a number of arguments and it's not easily discerned at runtime the number or the
types of those arguments unless the caller knows it ahead of time.  So just blindly calling methods discovered from dir() will most
likely result in a number of 'not enough arguments' errors.

A number of python modules provide some sort of naming convention to handle this issue, such as xmllib when you subclass the default
parser.  At each start of an element in the document it looks for a method called 'start_ELEMENTNAME' to invoke with the attributes
of the element.  This is a documented feature and how the caller of the method knows what and how many arguments to pass.

http://www.python.org/doc/current/lib/module-xmllib.html

os.path.walk() is a method that walks through the filesystem path.  At each directory it invokes a callable function, supplied by
the user, with three argument '(args, root, files)'.  Again, this is documented.  If the caller didn't know the callback expected
these three arguments an Error would occur.

http://www.python.org/doc/current/lib/module-os.path.html

I hope this clarifies things a bit.  My point is if you want to invoke random methods on an instance, the general pythonic way of
doing so is adopting a naming convention (such as xmllib or unittest) and only calling methods that adhere to the naming scheme.
It's the simplest and safest way to know the methods you are calling expect you to be calling them.

brian

> -----Original Message-----
> From: jyt...@li...
> [mailto:jyt...@li...]On Behalf Of Neil Benn
> Sent: Thursday, April 26, 2001 3:33 AM
> To: jyt...@li...
> Subject: RE: [Jython-users] Running auto-discovered methods
>
>
> Hello,
>
> 	That's great, thanks for posting that message Brian, I've looked at the
> link you posted but I can't seem to find any information about 'walking the
> path' as you mentioned to discover the arguments.  Would it be possible to
> elaborate on that point please?
>
> Much appreciated.
>
> Cheers,
>
> Neil
>
> -----Original Message-----
> From: jyt...@li...
> [mailto:jyt...@li...]On Behalf Of brian
> zimmer
> Sent: 21 April 2001 15:10
> To: Scott Knight; jython
> Subject: RE: [Jython-users] Running auto-discovered methods
>
>
> Scott,
>
> Here's an example of what I think you are looking for (it works equally well
> on a [Java|Python] [class|instance]:
>
> >>> class Test:
> ...  def go(self):
> ...   print "invoked go"
> ...  def went(self):
> ...   print "invoked went"
> ...
> >>> t = Test()
> >>> dir(t.__class__)
> ['__doc__', '__module__', 'go', 'went']
> >>> for a in dir(t.__class__):
> ...  if a[0:2] <> "__":
> ...   getattr(t, a)()
> ...
> invoked go
> invoked went
> >>>
>
> Notice the line:
>
>  getattr(t, a)()
>
> The built-in 'getattr' retuns a method bound on the instance t, similar to a
> Method object in Java.  In Java, to execute the Method,
> you would call:
>
>  m.invoke(instance, params)
>
> in Python you use the callable nature of objects and invoke it by:
>
>  m(params)
>
> since the method is already bound to the instance you don't need to pass it
> as a param as you do in Java since in Java the Method is
> always bound to the class, not an instance.
>
> For a more indepth look at when the method is bound to an instance and when
> to a class:
>
> >>> getattr(Test, "go")
> <unbound method Test.go>
> >>> getattr(t, "go")
> <method Test.go of Test instance at 3335245>
> >>> getattr(Test, "go")(t)
> invoked go
> >>> getattr(t, "go")()
> invoked go
> >>>
>
> Notice that when I get the method from the class, I need to pass an instance
> to invoke it, fulfilling the contract for sending a
> 'self' argument.
>
> Also note that neither go() or went() required arguments in my example.  If
> you really want to blindly invoke methods you'll need to
> figure out if they require arguments.  In general, the invoker knows the
> arguments required, such as the callback in os.path.walk().
>
> You might want to check out this as well:
>
> http://www.diveintopython.org/apihelper_getattr.html
>
> hope this helps,
>
> brian
>
> > -----Original Message-----
> > From: jyt...@li...
> > [mailto:jyt...@li...]On Behalf Of Scott
> > Knight
> > Sent: Saturday, April 21, 2001 7:52 AM
> > To: jython
> > Subject: [Jython-users] Running auto-discovered methods
> >
> >
> > A few days ago, Neil Benn queried "Self Describing Scripts in Jython"
> > and appears to be wanting to do something similar to what I need:
> > something analogous to Java's Reflection.  I can easily get a list of
> > the method names in a class.  I can't figure out how to invoke the
> > method given it's name in string form.  What I am really after is to
> > cycle through all of the method names in the list and run them all,
> > rather than type up a method that calls each of a couple hundred methods
> > statically.
> > --
> > Scott Knight  mailto:sc...@sc...
> > http://www.scottknight.com/
> >
> > _______________________________________________
> > Jython-users mailing list
> > Jyt...@li...
> > http://lists.sourceforge.net/lists/listinfo/jython-users
>
>
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