I have a user-defined function in python which I don't know how many arguments it has. i.e. 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;
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
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.
foobar accept), and not the count of invocation arguments (i.e. len(args) + 1).
*argsand find the length of that