[C++-sig] Need strategic advice designing Boost/Python program

[Ralf W. Grosse-Kunstleve]

> This, together with your point further down that you want to add python
> methods that are only ever used inside python make it sound as if
> a C++ base class derived from in python may be the best approach.

FWIW: there is a very powerful alternative to this: injecting.
For example:

class _histogram(boost.python.injector, ext.histogram):

  def show(self, f=None, prefix="", format_cutoffs="%.8g"):
    if (f is None): f = sys.stdout
    fmt = "%s" + format_cutoffs + " - " + format_cutoffs + ": %d"
    for info in self.slot_infos():
      print >> f, fmt % (prefix, info.low_cutoff, info.high_cutoff, info.n)

The boost.python.injector is a small utility class defined
in boost_adaptbx (adaptor toolbox), boost/python.py.

The difference is that objects of ext.histogram will have the
.show() method, even if they are created in C++.
I.e. even if you only have a C++ constructor, you can have pure
Python methods.

To give proper credit: this trick is due to David Abrahams. It was
one of the big aha moments in my life when he explained it to me.

Ralf

P.S.: you can also do it like this:

  def histogram_show(self, f=None, prefix="", format_cutoffs="%.8g"):
    # exact same code

  ext.histogram.show = histogram_show

but it doesn't look nearly as nice. You can also make it look nice with
decorators if you don't care about Python 2.2 and 2.3 compatibility.




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