...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Extending Wrapped Objects in Python |
Thanks to Python's flexibility, you can easily add new methods to a class, even after it was already created:
>>> class C(object): pass
>>>
>>> ##a regular function
>>> def C_str(self): return 'A C instance!'
>>>
>>> ##now we turn it in a member function
>>> C.__str__ = C_str
>>>
>>> c = C()
>>> print c
A C instance!
>>> C_str(c)
A C instance!
Yes, Python rox.
We can do the same with classes that were wrapped with Boost.Python. Suppose we have a class point in C++:
class point {...};
BOOST_PYTHON_MODULE(_geom)
{
class_<point>("point")...;
}
If we are using the technique from the previous session, Creating Packages, we can code directly into geom/__init__.py:
from _geom import *
##a regular function
def point_str(self):
return str((self.x, self.y))
##now we turn it into a member function
point.__str__ = point_str
All point instances created from C++ will also have this member function! This technique has several advantages:
You can even add a little syntactic sugar with the use of metaclasses. Let's create a special metaclass that "injects" methods in other classes.
##The one Boost.Python uses for all wrapped classes.
##You can use here any class exported by Boost instead of "point"
BoostPythonMetaclass = point.__class__
class injector(object):
class __metaclass__(BoostPythonMetaclass):
def __init__(self, name, bases, dict):
for b in bases:
if type(b) not in (self, type):
for k,v in dict.items():
setattr(b,k,v)
return type.__init__(self, name, bases, dict)
##inject some methods in the point foo
class more_point(injector, point):
def __repr__(self):
return 'Point(x=%s, y=%s)' % (self.x, self.y)
def foo(self):
print 'foo!'
Now let's see how it got:
>>> print point()
Point(x=10, y=10)
>>> point().foo()
foo!
Another useful idea is to replace constructors with factory functions:
_point = point
def point(x=0, y=0):
return _point(x, y)
In this simple case there is not much gained, but for constructurs with many overloads and/or arguments this is often a great simplification, again with virtually zero memory footprint and zero compile-time overhead for the keyword support.
Copyright © 2002-2003 David Abrahams
Copyright © 2002-2003 Joel de Guzman
Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
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