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python - How can I write a function to accept a Fraction object in SWIG?

问题描述

I'm writing an interface between Python and a our C++ video-processing library using SWIG. In python, I use the Fraction class to represent frame rate (e.g. NTFS24 = 24000/1001 FPS). The functionality in question is video transcoding, i.e. taking a video (or a stream of frames) input and producing a similar output. For that we need to specify the output (and sometimes input) frame rate.

Is there any way I can interface the Fraction class on the C++ (SWIG) side? From what I found around the Internet I should be able to pass a tuple to a std::pair<int,int> parameter, so that's my fallback plan, but is there a better way? Thanks!

标签： pythonc++swigfractions

解决方案

我将以下接口文件放在一起来说明包装 Fraction 的工作原理。最后我决定创建自己的分数结构来保存 C++ 端的分数，主要是因为它比使用std::pair<int, int>. （我认为一对整数也可以是 2D 坐标，或屏幕分辨率或许多其他类型，更强的类型对于重载分辨率等来说是更好的选择。）

%module test

%{
#include <iostream> // Just for testing....

static PyObject *fractions_module = NULL;
%}

%init %{
  // Import the module we want
  fractions_module = PyImport_ImportModule("fractions");
  assert(fractions_module);
  // TODO: we should call Py_DECREF(fractions_module) when our module gets unloaded
%}

%typemap(in) const Fraction& (Fraction tmp) {
  // Input typemap for fraction: duck-type on attrs numerator, denominator
  PyObject *numerator = PyObject_GetAttrString($input, "numerator");
  PyObject *denominator = PyObject_GetAttrString($input, "denominator");

  int err = SWIG_AsVal_int(numerator, &tmp.numerator);
  assert(SWIG_IsOK(err)); // TODO: proper error handling
  err = SWIG_AsVal_int(denominator, &tmp.denominator);
  assert(SWIG_IsOK(err)); // TODO: errors...

  Py_DECREF(numerator);
  Py_DECREF(denominator);

  $1 = &tmp;  
}

%typemap(out) Fraction {
  // Output typemap: pass two ints into fractions.Fraction() ctor
  $result = PyObject_CallMethod(fractions_module, "Fraction", "ii", $1.numerator, $1.denominator);
}

%inline %{
  struct Fraction {
    int numerator, denominator;
  };

  void fraction_in(const Fraction& fraction) {
    std::cout << fraction.numerator << "/" << fraction.denominator << "\n";
  }

  Fraction fraction_out() {
    Fraction f = {100, 1};
    return f;
  }
%}

大多数情况下，这只是两个类型映射 - 一个用于 C++ 函数的输入，一个用于输出。他们从输入对象的分子和分母属性构造一个临时的 C++ 分数，并fractions.Fraction分别从我们的 C++ 构造一个 Python 对象。将它们调整为其他类似的小数类型应该相当简单。

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