Sending Python function as Boost.Function argument

Question

Things are getting complicated in my world of trying to mesh Python code with my C++.

Essentially, I want to be able to assign a callback function to be used after an HTTP call receives a response, and I want to be able to do this from either C++ or Python.

In other words, I want to be able to call this from C++:

http.get_asyc("www.google.ca", [&](int a) { std::cout << "response recieved: " << a << std::endl; });

and this from Python:

def f(r):
    print str.format('response recieved: {}', r)

http.get_async('www.google.ca', f)

I have set up a demo on Coliru that shows exactly what I'm trying to accomplish. Here is the code and the error that I am getting:

C++

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

struct http_manager
{
    void get_async(std::string url, boost::function<void(int)> on_response)
    {
        if (on_response)
        {
            on_response(42);
        }
    }
} http;

BOOST_PYTHON_MODULE(example)
{
    boost::python::class_<http_manager>("HttpManager", boost::python::no_init)
        .def("get_async", &http_manager::get_async);

    boost::python::scope().attr("http") = boost::ref(http);
}

Python

import example
def f(r):
    print r
example.http.get_async('www.google.ca', f)

Error

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
Boost.Python.ArgumentError: Python argument types in
    HttpManager.get_async(HttpManager, str, function)
did not match C++ signature:
    get_async(http_manager {lvalue}, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, boost::function<void (int)>)

I'm not sure why the function is not being converted to a boost::function automatically.

I have asked a vaguely similar question on SO before and got an amazing answer. I also wonder if a similar method in the answer given there could be applied to this use-case as well.

Thank you very much for any and all support!

Answer 1

One solution is to add an overload function:

void get_async(std::string url, boost::python::object obj)
{
    if (PyCallable_Check(obj.ptr()))
        get_async(url, static_cast<boost::function<void(int)>>(obj));
}

Then expose just this specific overload:

.def("get_async", static_cast<void (http_manager::*)(std::string, boost::python::object)>(&http_manager::get_async))

Or if your don't want to pollute your main class with python stuff then you could create a wrapper class. Things look much cleaner then too:

struct http_manager_wrapper : http_manager
{
    void get_async(std::string url, boost::python::object obj)
    {
        if (PyCallable_Check(obj.ptr()))
            http_manager::get_async(url, obj);
    }

} http_wrapper;

BOOST_PYTHON_MODULE(example)
{
    boost::python::class_<http_manager_wrapper>("HttpManager", boost::python::no_init)
        .def("get_async", &http_manager_wrapper::get_async);

    boost::python::scope().attr("http") = boost::ref(http_wrapper);
}

---

Update: Another option is to use a python callable to boost function converter. This will address the singleton problem and won't require changes to the main class.

struct http_manager
{
    void get_async(std::string url, boost::function<void(int)> on_response)
    {
        if (on_response)
        {
            on_response(42);
        }
    }
} http;

struct BoostFunc_from_Python_Callable
{
    BoostFunc_from_Python_Callable()
    {
        boost::python::converter::registry::push_back(&convertible, &construct, boost::python::type_id< boost::function< void(int) > >());
    }

    static void* convertible(PyObject* obj_ptr)
    {
        if (!PyCallable_Check(obj_ptr)) 
            return 0;
        return obj_ptr;
    }

    static void construct(PyObject* obj_ptr, boost::python::converter::rvalue_from_python_stage1_data* data)
    {
        boost::python::object callable(boost::python::handle<>(boost::python::borrowed(obj_ptr)));
        void* storage = ((boost::python::converter::rvalue_from_python_storage< boost::function< void(int) > >*) data)->storage.bytes;
        new (storage)boost::function< void(int) >(callable);
        data->convertible = storage;
    }
};

BOOST_PYTHON_MODULE(example)
{
    // Register function converter
    BoostFunc_from_Python_Callable();

    boost::python::class_<http_manager>("HttpManager", boost::python::no_init)
        .def("get_async", &http_manager::get_async);

    boost::python::scope().attr("http") = boost::ref(http);
}

Answer 2

When a function that has been exposed through Boost.Python is invoked, Boost.Python will query its registry to locate a suitable from-Python converter for each of the caller's arguments based on the desired C++ type. If a converter is found that knows how to convert from the Python object to the C++ object, then it will use the converter to construct the C++ object. If no suitable converters are found, then Boost.Python will raise an ArgumentError exception.

The from-Python converters are registered:

• automatically for types supported by Boost.Python, such as int and std::string
• implicitly for types exposed by boost::python::class<T>. By default, the resulting Python class will hold an embedded instance of a T C++ object, and register to-Python and from-Python converters for the Python class and type T, using the embedded instance.
• explicitly via boost::python::converter::registry::push_back()

The steps of testing for convertibility and constructing an object occur in two distinct steps. As no from-Python converter has been registered for boost::function<void(int)>, Boost.Python will raise an ArgumentError exception. Boost.Python will not attempt construct the boost::function<void(int)> object, despite boost::function<void(int)> being constructible from a boost::python::object.

---

To resolve this, consider using an shim function to defer the construction of boost::function<void(int)> until after the boost::python::object has passed through the Boost.Python layer:

void http_manager_get_async_aux(
  http_manager& self, std::string url, boost::python::object on_response)
{
  return self.get_async(url, on_response);
}

...

BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;
  python::class_<http_manager>("HttpManager", python::no_init)
    .def("get_async", &http_manager_get_async_aux);

  ...
}

Here is a complete example demonstrating this approach:

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

struct http_manager
{
  void get_async(std::string url, boost::function<void(int)> on_response)
  {
    if (on_response)
    {
      on_response(42);
    }
  }
} http;

void http_manager_get_async_aux(
  http_manager& self, std::string url, boost::python::object on_response)
{
  return self.get_async(url, on_response);
}

BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;
  python::class_<http_manager>("HttpManager", python::no_init)
    .def("get_async", &http_manager_get_async_aux);

  python::scope().attr("http") = boost::ref(http);
}

Interactive usage:

>>> import example
>>> result = 0
>>> def f(r):
...     global result
...     result = r
...
>>> assert(result == 0)
>>> example.http.get_async('www.google.com', f)
>>> assert(result == 42)
>>> try:
...     example.http.get_async('www.google.com', 42)
...     assert(False)
... except TypeError:
...    pass
...

An alternative approach is to explicitly register a from-Python converter for boost::function<void(int)>. This has the benefit that all functions exposed through Boost.Python can use the converter (e.g. one would not need to write a shim for each function). However, a convert would need to be registered for each C++ type. Here is an example demonstrating explicitly registering a custom converter for boost::function<void(int)> and boost::function<void(std::string)>:

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

struct http_manager
{
  void get_async(std::string url, boost::function<void(int)> on_response)
  {
    if (on_response)
    {
      on_response(42);
    }
  }
} http;

/// @brief Type that allows for registration of conversions from
///        python iterable types.
struct function_converter
{
  /// @note Registers converter from a python callable type to the
  ///       provided type.
  template <typename FunctionSig>
  function_converter&
  from_python()
  {
    boost::python::converter::registry::push_back(
      &function_converter::convertible,
      &function_converter::construct<FunctionSig>,
      boost::python::type_id<boost::function<FunctionSig>>());

    // Support chaining.
    return *this;
  }

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

  /// @brief Convert callable PyObject to a C++ boost::function.
  template <typename FunctionSig>
  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 boost::function<FunctionSig> functor_type;
    typedef python::converter::rvalue_from_python_storage<functor_type>
                                                                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.
    new (storage) functor_type(python::object(handle));
    data->convertible = storage;
  }
};

BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;
  python::class_<http_manager>("HttpManager", python::no_init)
    .def("get_async", &http_manager::get_async);

  python::scope().attr("http") = boost::ref(http);

  // Enable conversions for boost::function.
  function_converter()
    .from_python<void(int)>()
    // Chaining is supported, so the following would enable
    // another conversion.
    .from_python<void(std::string)>()
    ;
}