Boost.Python

Contents

Introduction

Classes

Class Templatefrom_python

Class Template from_python synopsis

Class Template from_python constructor

Class Template from_python observer functions

Example

Introduction

<boost/python/from_python.hpp> introduces a class template from_python<T> for extracting a C++ object of type T from a Python object.

Classes

Class Template from_python<class T>

from_python<T> is the type used internally by Boost.Python to extract C++ function arguments from a Python argument tuple when calling a wrapped function. It can also be used directly to make similar conversions in other contexts.

Class Template from_python synopsis

namespace boost { namespace python
{
   template <class T>
   struct from_python : private boost::noncopyable // Exposition only.
       // from_python<T> meets the NonCopyable requirements
   {
      from_python(PyObject*);
      bool convertible() const;
      convertible-to-T operator()(PyObject*) const;
   };
}

Class Template from_python constructor

from_python(PyObject* p);

Requires: p != 0

Effects: Constructs a from_python object suitable for extracting a C++ object of type T from p.

Class Template from_python observer functions

bool convertible() const;

Returns: false if the conversion cannot succeed. This indicates that either:

1. No from_python_converter was registered for T, or
2. any such converter rejected the constructor argument p by returning 0 from its convertible() function

Note that conversion may still fail in operator() due to an exception.

Throws: nothing

Rationale: Because from_python<> is used in overload resolution, and throwing an exception can be slow, it is useful to be able to rule out a broad class of unsuccessful conversions without throwing an exception.

convertible-to-T operator()(PyObject* p) const;

Requires: *p refers to the same object which was passed to the constructor, and convertible() returns true.

Effects: performs the conversion

Returns: an object convertible to T.

Example

#include <string>
#include <boost/python/from_python.hpp>

// If a std::string can be extracted from p, return its
// length. Otherwise, return 0.
std::size_t length_if_string(PyObject* p)
{
   from_python<std::string> converter(p);
   if (!converter.convertible())
      return 0;
   else
      return converter().size();
}

Revised 05 November, 2001

© Copyright Dave Abrahams 2002. All Rights Reserved.