Boost.Python自定义转换器

Question

我有一个类作为参数(二进制文件内容).

我想将python'str'类型转换为unsigned char的向量,但仅用于我的一个类方法.

BOOST_PYTHON_MODULE(hello) {  class_<Hello>("Hello").
     // This method takes a string as parameter and print it
     .def("printChar", &Hello::printChar)
     // This method takes a vector<unsigned char> parameter
     .def("storeFile", &Hello::storeFile) }

使用自定义转换器似乎是我需要的但是如果我修改我的boost :: python :: converter :: registry它将修改我对printChar的所有调用以及所有传递string的python方法将参数转换为vector.

如何注册每个方法的转换器？

Answer 1

这个问题有两种方法:

• 导出一个辅助函数,Hello.storeFile它接受boost::python::str,std::vector<unsigned char>从字符串构造,并委托给C++ Hello::storeFile成员函数.
• 编写自定义转换器.虽然转换器无法基于每个功能进行注册,但它们的范围非常合理,因为它们不会执行任何意外的转换.这种方法通常提供更多的可重用性.

辅助功能

使用辅助函数不会影响任何其他导出函数.因此,python字符串之间的转换std::vector<unsigned char>只会发生Hello.storeFile.

void Hello_storeFile(Hello& self, boost::python::str str)
{
  std::cout << "Hello_storeFile" << std::endl;
  // Obtain a handle to the string.
  const char* begin = PyString_AsString(str.ptr());
  // Delegate to Hello::storeFile().
  self.storeFile(std::vector<unsigned char>(begin, begin + len(str)));
}

...

BOOST_PYTHON_MODULE(hello)
{
  namespace python = boost::python;

  python::class_<Hello>("Hello")
    // This method takes a string as parameter and print it
    .def("printChar", &Hello::printChar)
    // This method takes a vector<unsigned char> parameter
    .def("storeFile", &Hello_storeFile)
    ;
}

定制转换器

转换器注册有三个部分:

• 检查a PyObject是否可转换的函数.返回NULL表示PyObject不能使用已注册的转换器.
• 构造函数,用于构造C++类型PyObject.只有在converter(PyObject)不返回时才会调用此函数NULL.
• 将构造的C++类型.

因此,对于给定的C++类型,如果converter(PyObject)返回非NULL值,construct(PyObject)则将创建C++类型.C++类型充当注册表的键,因此Boost.Python不应执行非预期的转换.

在问题的上下文中,我们想要一个转换器,std::vector<unsigned char>其中converter(PyObject)返回非NULLif ,如果PyObject是a PyString,converter(PyObject)将用于PyObject创建和填充std::vector<unsigned char>.仅当导出的C++函数具有std::vector<unsigned char>(或const引用)参数且python提供的参数是字符串时,才会发生此转换.因此,此自定义转换器不会影响具有std::string参数的导出函数.

这是一个完整的例子.我选择使转换器通用,以允许从python字符串构造多个类型.凭借其链接支持,它应该具有与其他Boost.Python类型相同的感觉.

#include <iostream>
#include <list>
#include <string>
#include <vector>

#include <boost/foreach.hpp>
#include <boost/python.hpp>

class Hello
{
public:
  void printChar(const std::string& str)
  {
    std::cout << "printChar: " << str << std::endl;
  }

  void storeFile(const std::vector<unsigned char>& data)
  {
    std::cout << "storeFile: " << data.size() << ": ";
    BOOST_FOREACH(const unsigned char& c, data)
      std::cout << c;
    std::cout << std::endl;
  }
};

/// @brief Type that allows for conversions of python strings to
//         vectors.
struct pystring_converter
{

  /// @note Registers converter from a python interable type to the
  ///       provided type.
  template <typename Container>
  pystring_converter&
  from_python()
  {
    boost::python::converter::registry::push_back(
      &pystring_converter::convertible,
      &pystring_converter::construct<Container>,
      boost::python::type_id<Container>());
    return *this;
  }

  /// @brief Check if PyObject is a string.
  static void* convertible(PyObject* object)
  {
    return PyString_Check(object) ? object : NULL;
  }

  /// @brief Convert PyString to Container.
  ///
  /// Container Concept requirements:
  ///
  ///   * Container::value_type is CopyConstructable from char.
  ///   * Container can be constructed and populated with two iterators.
  ///     I.e. Container(begin, end)
  template <typename Container>
  static void construct(
    PyObject* object,
    boost::python::converter::rvalue_from_python_stage1_data* data)
  {
    namespace python = boost::python;
    // Object is a borrowed reference, so create a handle indicting it is
    // borrowed for proper reference counting.
    python::handle<> handle(python::borrowed(object));

    // Obtain a handle to the memory block that the converter has allocated
    // for the C++ type.
    typedef python::converter::rvalue_from_python_storage<Container>
                                                                 storage_type;
    void* storage = reinterpret_cast<storage_type*>(data)->storage.bytes;

    // Allocate the C++ type into the converter's memory block, and assign
    // its handle to the converter's convertible variable.  The C++
    // container is populated by passing the begin and end iterators of
    // the python object to the container's constructor.
    const char* begin = PyString_AsString(object);
    data->convertible = new (storage) Container(
      begin,                          // begin
      begin + PyString_Size(object)); // end
  }
};

BOOST_PYTHON_MODULE(hello)
{
  namespace python = boost::python;

  // Register PyString conversions.
  pystring_converter()
    .from_python<std::vector<unsigned char> >()
    .from_python<std::list<char> >()
    ;

  python::class_<Hello>("Hello")
    // This method takes a string as parameter and print it
    .def("printChar", &Hello::printChar)
    // This method takes a vector<unsigned char> parameter
    .def("storeFile", &Hello::storeFile)
    ;
}

以及示例用法:

>>> from hello import Hello
>>> h = Hello()
>>> h.printChar('abc')
printChar: abc
>>> h.storeFile('def')
storeFile: 3: def
>>> h.storeFile([c for c in 'def'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Boost.Python.ArgumentError: Python argument types in
    Hello.storeFile(Hello, list)
did not match C++ signature:
    storeFile(Hello {lvalue}, std::vector<unsigned char, 
                                          std::allocator<unsigned char> >)

有关自定义转换器和C++容器的更多信息,请考虑阅读此答案.