使用Boost-Python查找函数具有的参数数量

Question

I have a user-defined function in python which I don't know how many arguments it has. ie It can be

def f1(x,y,z):
  return  sin(x)*cos(y)*tan(z)

or

def f1(x):
  return  sin(x)

How I can use Boost-Python to find how many arguments the function has? If I just had 1 function argument, I could always evaluate like this:

  bp::object f1 = main_dict["f1"];
  std::cout << bp::extract<double>(f1(0.0)) << std::endl;

Answer 1

For user-defined functions in python, it is possible to extract the arity of a function through the func_code special attribute. This attribute represents the compiled function body, and provides a co_argcount attribute indicating the function's arity. For more information and other possible approaches, consider reading this question.

Ignoring error checking, the Boost.Python implementation becomes fairly trivial:

boost::python::extract<std::size_t>(fn.attr("func_code").attr("co_argcount"));

---

Here is a complete brief example:

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

void print_arity(boost::python::object fn)
{
  std::size_t arity = boost::python::extract<std::size_t>(
                        fn.attr("func_code").attr("co_argcount"));
  std::cout << arity << std::endl;
}

BOOST_PYTHON_MODULE(example)
{
  def("print_arity", &print_arity);
}

And its usage:

>>> from example import print_arity
>>> def f1(x,y,z): pass
... 
>>> print_arity(f1)
3
>>> def f1(x): pass
... 
>>> print_arity(f1)
1

Answer 2

def foobar(x, *args):
    return len(args) + 1

The *args in the parameter list assigns all extra arguments not explicitly defined in the paramater list to a list args (this name is user-defined). Assuming the function must have at least one parameter, the above function will return the number of arguments.

Disclaimer: I have no experience whatsoever with Boost.Python. This answer is merely the implementation in Python and transferring the return value into C++ is up to the user.