Boost C++ Libraries

Derived Object types

Boost.Python comes with a set of derived object types corresponding to that of Python's:

• list
• dict
• tuple
• str
• long_
• enum

These derived object types act like real Python types. For instance:

    str(1) ==> "1"

Wherever appropriate, a particular derived object has corresponding Python type's methods. For instance, dict has a keys() method:

    d.keys()

make_tuple is provided for declaring tuple literals. Example:

    make_tuple(123, 'D', "Hello, World", 0.0);

In C++, when Boost.Python objects are used as arguments to functions, subtype matching is required. For example, when a function f, as declared below, is wrapped, it will only accept instances of Python's str type and subtypes.

    void f(str name)
    {
        object n2 = name.attr("upper")();   // NAME = name.upper()
        str NAME = name.upper();            // better
        object msg = "%s is bigger than %s" % make_tuple(NAME,name);
    }

In finer detail:

    str NAME = name.upper();

Illustrates that we provide versions of the str type's methods as C++ member functions.

    object msg = "%s is bigger than %s" % make_tuple(NAME,name);

Demonstrates that you can write the C++ equivalent of "format" % x,y,z in Python, which is useful since there's no easy way to do that in std C++.

Beware the common pitfall of forgetting that the constructors of most of Python's mutable types make copies, just as in Python.

Python:

    >>> d = dict(x.__dict__)     ##copies x.__dict__
    >>> d['whatever']            ##modifies the copy

C++:

    dict d(x.attr("__dict__"));  ##copies x.__dict__
    d['whatever'] = 3;           ##modifies the copy

class_<T> as objects

Due to the dynamic nature of Boost.Python objects, any class_<T> may also be one of these types! The following code snippet wraps the class (type) object.

We can use this to create wrapped instances. Example:

    object vec345 = (
        class_<Vec2>("Vec2", init<double, double>())
            .def_readonly("length", &Point::length)
            .def_readonly("angle", &Point::angle)
        )(3.0, 4.0);

    assert(vec345.attr("length") == 5.0);

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 is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.