Invoking Python from Python

[Mike Meyer]

claird at lairds.us (Cameron Laird) writes:
> I'll rein myself in and suggest an even easier introduction
> to this subject:  configuration files.  RARELY is the correct
> answer to create a new syntax, although many development
> organizations give the impression that's their first choice.
> ".ini"-speak is a safe-enough choice.  Most interesting,
> though, is to interpret Python or some subset as a configu-
> ration specification, so that one has immediately not just
>     HOME = "/some/folder"
>     STEP_LIMIT = 16
> but 
>     pool_size = cpu_count * 30
> and even
>     if today == "Sunday":
>         total_process_maximum = 8
> available in the configuration language.  Neat, eh?  

I once carried this a step further, and used methodless classes as a
configuration mechanism:

class PlainFoo:
      # Values for a plain foo

class FancyFoo(PlainFoo):
      # Overrides for just the things that are different

The program that used this created needed lots of objects, in a
variety of different flavers that were literally specified as "Just
like PlainFoo, but with ...". Doing it this way made configuring
things trivial.

At the time, I attached "configuration variables" to instances. If I
were going to do it today, I'd look into making the parent classes of
the class that implements Foo dynanmic.

plwm (an X11 window manager - sort of - built in top of python-xlib)
carries this another step further. You configure your window manager
by creating a subclass of the WindowManager (or other) class that
mixes in the features you want, and sets the attributes to control
specific features.

It's very flexible - but at this point, the "configuration file" is a
Python program, and not really suitable to use by non-programmers.

> But if configuration is *that* powerful, then it can also
> do great damage.  How does one make Python interpretation safe?
> That's a subject for another day.

We're all waiting for this, somewhat impatiently :-).

      <mike
-- 
Mike Meyer <mwm at mired.org>			http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.