Getting argument list in a Boost:Python function

Question

In CPython we can get the argument list of a function by following methods. function name is 'aMethod'

import inspect
inspect.getargspec(aMethod)

or

aMethod.func_code.co_varnames

How can I achieve the same thing for a Boost:Python function? I get the following error when I use these methods in for those.

for the first method TypeError: is not a Python function

for the second method AttributeError: 'aMethod' object has no attribute 'func_code'

Answer 1

When accessing a Python attribute on a boost::python::object, use the attr member function. For example:

aMethod.func_code.co_varnames

would become

aMethod.attr("func_code").attr("co_varnames")

---

Here is a complete example.

#include <iostream>
#include <vector>

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

void print_varnames(boost::python::object fn)
{
  namespace python = boost::python;
  typedef python::stl_input_iterator<std::string> iterator;

  std::vector<std::string> var_names(
    iterator(fn.attr("func_code").attr("co_varnames")),
    iterator());

  BOOST_FOREACH(const std::string& varname, var_names)
    std::cout << varname << std::endl;
}

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

Usage:

>>> def test1(a,b,c): pass
... 
>>> def test2(spam, eggs): pass
... 
>>> def test3(): pass
... 
>>> from example import print_varnames
>>> print_varnames(test1)
a
b
c
>>> print_varnames(test2)
spam
eggs
>>> print_varnames(test3)
>>> 

Answer 2

Careful: This approach will print out all variables in a function, not just its parameters. To wit:

def test4(a, b):
    c = a + b

>>> test4.__code__.co_varnames
('a', 'b', 'c')

If you really need just the function parameters, just use the inspect module:

// given f as callable boost::python::object
auto inspect = python::import("inspect");
auto argspec = inspect.attr("getargspec")(f);
// returns args in argspec[0] (and varargs in [1], keywords in [2], and defaults in [3]).