Boost C++ Libraries

“...one of the most highly regarded and expertly designed C++ library projects in the world.” — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Boost.Python

Headers <boost/python/class.hpp>, <boost/python/class_fwd.hpp>

Contents

Introduction
Classes
Class template class_
Class class_ synopsis
Class class_ constructors
Class class_ modifier functions
Class template bases
Class template bases synopsis
Example(s)

Introduction

<boost/python/class.hpp> defines the interface through which users expose their C++ classes to Python. It declares the class_ class template, which is parameterized on the class type being exposed. It also exposes the init, optional and bases utility class templates, which are used in conjunction with class_.

<boost/python/class_fwd.hpp> contains a forward declaration of the class_ class template.

Classes

Class template class_<T, Bases, HeldType, NonCopyable>

Creates a Python class associated with the C++ type passed as its first parameter. Although it has four template parameters, only the first one is required. The three optional arguments can actually be supplied in any order; Boost.Python determines the role of the argument from its type.

Template Parameter Requirements Semantics Default
T A class type. The class being wrapped
Bases A specialization of bases<...> which specifies previously-exposed C++ base classes of T[1]. Registers from_python conversions from wrapped T instances to each of its exposed direct and indirect bases. For each polymorphic base B, registers conversions from indirectly-held wrapped B instances to T. bases<>
HeldType Must be T, a class derived from T, or a Dereferenceable type for which pointee<HeldType>::type is T or a class derived from T. Specifies the type that is actually embedded in a Python object wrapping a T instance when T's constructor is called or when a T or T* is converted to Python without the use of ptr, ref, or Call Policies such as return_internal_reference. More details below. T
NonCopyable If supplied, must be boost::noncopyable. Suppresses automatic registration of to_python conversions which copy T instances. Required when T has no publicly-accessible copy constructor. An unspecified type other than boost::noncopyable.

HeldType Semantics

  1. If HeldType is derived from T, its exposed constructor(s) must accept an initial PyObject* argument which refers back to the Python object that contains the HeldType instance, as shown in this example. This argument is not included in the init-expression passed to def(init_expr), below, nor is it passed explicitly by users when Python instances of T are created. This idiom allows C++ virtual functions which will be overridden in Python to access the Python object so the Python method can be invoked. Boost.Python automatically registers additional converters which allow wrapped instances of T to be passed to wrapped C++ functions expecting HeldType arguments.
  2. Because Boost.Python will always allow wrapped instances of T to be passed in place of HeldType arguments, specifying a smart pointer for HeldType allows users to pass Python T instances where a smart pointer-to-T is expected. Smart pointers such as std::auto_ptr<> or boost::shared_ptr<> which contain a nested type element_type designating the referent type are automatically supported; additional smart pointer types can be supported by specializing pointee<HeldType>.
  3. As in case 1 above, when HeldType is a smart pointer to a class derived from T, the initial PyObject* argument must be supplied by all of HeldType's exposed constructors.
  4. Except in cases 1 and 3, users may optionally specify that T itself gets initialized with a similar initial PyObject* argument by specializing has_back_reference<T>.

Class template class_ synopsis

namespace boost { namespace python
{
  template <class T
      , class Bases = bases<>
            , class HeldType = T
            , class NonCopyable = unspecified
           >
  class class_ : public object
  {
    // Constructors with default __init__
    class_(char const* name);
    class_(char const* name, char const* docstring);

    // Constructors, specifying non-default __init__
    template <class Init>
    class_(char const* name, Init);
    template <class Init>
    class_(char const* name, char const* docstring, Init);

    // Exposing additional __init__ functions
    template <class Init>
    class_& def(Init);

    // defining methods
    template <class F>
    class_& def(char const* name, F f);
    template <class Fn, class A1>
    class_& def(char const* name, Fn fn, A1 const&);
    template <class Fn, class A1, class A2>
    class_& def(char const* name, Fn fn, A1 const&, A2 const&);
    template <class Fn, class A1, class A2, class A3>
    class_& def(char const* name, Fn fn, A1 const&, A2 const&, A3 const&);

    // declaring method as static
    class_& staticmethod(char const* name);
    
    // exposing operators
    template <unspecified>
    class_& def(detail::operator_<unspecified>);

    // Raw attribute modification
    template <class U>
    class_& setattr(char const* name, U const&);

    // exposing data members
    template <class D>
    class_& def_readonly(char const* name, D T::*pm);

    template <class D>
    class_& def_readwrite(char const* name, D T::*pm);

    // exposing static data members
    template <class D>
    class_& def_readonly(char const* name, D const& d);
    template <class D>
    class_& def_readwrite(char const* name, D& d);

    // property creation
    template <class Get>
    void add_property(char const* name, Get const& fget, char const* doc=0);
    template <class Get, class Set>
    void add_property(
        char const* name, Get const& fget, Set const& fset, char const* doc=0);

    template <class Get>
    void add_static_property(char const* name, Get const& fget);
    template <class Get, class Set>
    void add_static_property(char const* name, Get const& fget, Set const& fset);

    // pickle support
    template <typename PickleSuite>
    self& def_pickle(PickleSuite const&);
    self& enable_pickling();
  };
}}

Class template class_ constructors

class_(char const* name);
class_(char const* name, char const* docstring);
template <class Init>
class_(char const* name, Init init_spec);
template <class Init>
class_(char const* name, char const* docstring, Init init_spec);
Requires: name is an ntbs which conforms to Python's identifier naming rules. If docstring is supplied, it must be an ntbs. If init_spec is supplied, it must be either the special enumeration constant no_init or an init-expression compatible with T.
Effects: Constructs a class_ object holding a Boost.Python extension class named name. The named attribute of the current scope is bound to the new extension class.
  • If supplied, the value of docstring is bound to the __doc__ attribute of the extension class.
  • If init_spec is no_init, a special __init__ function is generated which always raises a Python exception. Otherwise, this->def(init_spec) is called.
  • If init_spec is not supplied, this->def(init<>()) is called.
Rationale:Allowing the user to specify constructor arguments in the class_<> constructor helps her to avoid the common run-time errors which result from invoking wrapped member functions without having exposed an __init__ function which creates the requisite T instance. Types which are not default-constructible will cause a compile-time error unless Init is supplied. The user must always supply name as there is currently no portable method to derive the text of the class name from its type.

Class template class_ modifier functions

template <class Init>
class_& def(Init init_expr);
Requires: init_expr is the result of an init-expression compatible with T.
Effects: For each valid prefix P of Init, adds an __init__(...) function overload to the extension class accepting P as arguments. Each overload generated constructs an object of HeldType according to the semantics described above, using a copy of init_expr's call policies. If the longest valid prefix of Init contains N types and init_expr holds M keywords, an initial sequence of the keywords are used for all but the first N - M arguments of each overload.
Returns: *this
Rationale: Allows users to easily expose a class' constructor to Python.

template <class F>
class_& def(char const* name, Fn fn);
template <class Fn, class A1>
class_& def(char const* name, Fn fn, A1 const& a1);
template <class Fn, class A1, class A2>
class_& def(char const* name, Fn fn, A1 const& a1, A2 const& a2);
template <class Fn, class A1, class A2, class A3>
class_& def(char const* name, Fn fn, A1 const& a1, A2 const& a2, A3 const& a3);
Requires: name is an ntbs which conforms to Python's identifier naming rules.
  • If a1 is the result of an overload-dispatch-expression, only the second form is allowed and fn must be a pointer to function or pointer to member function whose arity is the same as A1's maximum arity.
    Effects: For each prefix P of Fn's sequence of argument types, beginning with the one whose length is A1's minimum arity, adds a name(...) method overload to the extension class. Each overload generated invokes a1's call-expression with P, using a copy of a1's call policies. If the longest valid prefix of A1 contains N types and a1 holds M keywords, an initial sequence of the keywords are used for all but the first N - M arguments of each overload.
  • Otherwise, a single method overload is built around fn, which must not be null:
    • If fn is a function pointer, its first argument must be of the form U, U cv&, U cv*, or U cv* const&, where T* is convertible to U*, and a1-a3, if supplied, may be selected in any order from the table below.
    • Otherwise, if fn is a member function pointer, its target must be T or one of its public base classes, and a1-a3, if supplied, may be selected in any order from the table below.
    • Otherwise, Fn must be [derived from] object, and a1-a2, if supplied, may be selcted in any order from the first two rows of the table below. To be useful, fn should be callable.
    Memnonic Name Requirements/Type properties Effects
    docstring Any ntbs. Value will be bound to the __doc__ attribute of the resulting method overload. If an earlier overload supplied a docstring, two newline characters and the new docstring are appended to it.
    policies A model of CallPolicies A copy will be used as the call policies of the resulting method overload.
    keywords The result of a keyword-expression specifying no more arguments than the arity of fn. A copy will be used as the call policies of the resulting method overload.
Returns: *this
class_& staticmethod(char const* name);
Requires: name is an ntbs which conforms to Python's identifier naming rules, and corresponds to a method whose overloads have all been defined.
Effects: Replaces the existing named attribute x with the result of invoking staticmethod(x) in Python. Specifies that the corresponding method is static and therefore no object instance will be passed to it. This is equivalent to the Python statement:
setattr(self, name, staticmethod(getattr(self, name)))
Note: Attempting to invoke def(name,...) after invoking staticmethod(name) will raise a RuntimeError.
Returns: *this

template <unspecified>
class_& def(detail::operator_<unspecified>);
Effects: Adds a Python special method as described here.
Returns: *this
template <class U>
class_& setattr(char const* name, U const& u);
Requires: name is an ntbs which conforms to Python's identifier naming rules.
Effects: Converts u to Python and adds it to the attribute dictionary of the extension class:
PyObject_SetAttrString(this->ptr(), name, object(u).ptr());
Returns: *this

template <class Get>
void add_property(char const* name, Get const& fget, char const* doc=0);
template <class Get, class Set>
void add_property(
        char const* name, Get const& fget, Set const& fset, char const* doc=0);
Requires: name is an ntbs which conform to Python's identifier naming rules.
Effects: Creates a new Python property class instance, passing object(fget) (and object(fset) in the second form) with an (optional) docstring doc to its constructor, then adds that property to the Python class object under construction with the given attribute name.
Returns: *this
Rationale: Allows users to easily expose functions that can be invoked from Python with attribute access syntax.

template <class Get>
void add_static_property(char const* name, Get const& fget);
template <class Get, class Set>
void add_static_property(char const* name, Get const& fget, Set const& fset);
Requires: name is an ntbs which conforms to Python's identifier naming rules.
Effects: Creates a Boost.Python.StaticProperty object, passing object(fget) (and object(fset) in the second form) to its constructor, then adds that property to the Python class under construction with the given attribute name. StaticProperty is a special subclass of Python's property class which can be called without an initial self argument.
Returns: *this
Rationale: Allows users to easily expose functions that can be invoked from Python with static attribute access syntax.

template <class D>
class_& def_readonly(char const* name, D T::*pm, char const* doc=0);
template <class D>
class_& def_readonly(char const* name, D const& d);
Requires: name is an ntbs which conforms to Python's identifier naming rules. doc is also an ntbs.
Effects:
this->add_property(name, make_getter(pm), doc);
and 
this->add_static_property(name, make_getter(d));
respectively.

Returns: *this
Rationale: Allows users to easily expose a class' data member or free variable such that it can be inspected from Python with a natural syntax.
template <class D>
class_& def_readwrite(char const* name, D T::*pm, char const* doc=0);
template <class D>
class_& def_readwrite(char const* name, D& d);
Effects:
this->add_property(name, make_getter(pm), make_setter(pm), doc);
and 
this->add_static_property(name, make_getter(d), make_setter(d));
respectively.

Returns: *this
Rationale: Allows users to easily expose a class' data or free variable member such that it can be inspected and set from Python with a natural syntax.
template <typename PickleSuite>
class_& def_pickle(PickleSuite const&);
Requires: PickleSuite must be publically derived from pickle_suite.
Effects: Defines a legal combination of the special attributes and methods: __getinitargs__, __getstate__, __setstate__, __getstate_manages_dict__, __safe_for_unpickling__, __reduce__
Returns: *this
Rationale: Provides an easy to use high-level interface for establishing complete pickle support for the wrapped class. The user is protected by compile-time consistency checks.

class_& enable_pickling();
Effects: Defines the __reduce__ method and the __safe_for_unpickling__ attribute.
Returns: *this
Rationale: Light-weight alternative to def_pickle(). Enables implementation of pickle support from Python.

Class template bases<T1, T2,...TN>

An MPL sequence which can be used in class_<...> instantiations indicate a list of base classes.

Class template bases synopsis

namespace boost { namespace python
{
  template <T1 = unspecified,...Tn = unspecified>
  struct bases
  {};
}}

Example(s)

Given a C++ class declaration:

class Foo : public Bar, public Baz
{
 public:
   Foo(int x, char const* y);
   Foo(double);

   std::string const& name() { return m_name; }
   void name(char const*);

   double value; // public data
 private:
   ...
};
A corresponding Boost.Python extension class can be created with: 
using namespace boost::python;

class_<Foo,bases<Bar,Baz> >("Foo",
          "This is Foo's docstring."
          "It describes our Foo extension class",

          init<int,char const*>(args("x","y"), "__init__ docstring")
          )
   .def(init<double>())
   .def("get_name", &Foo::get_name, return_internal_reference<>())
   .def("set_name", &Foo::set_name)
   .def_readwrite("value", &Foo::value)
   ;
[1] By "previously-exposed" we mean that the for each B in bases, an instance of class_<B, ...> must have already been constructed. 
class_<Base>("Base");
class_<Derived, bases<Base> >("Derived");
Revised 1 November, 2005

© Copyright Dave Abrahams 2002.