Writes the python executable definition for interacting with ctypes.

        :arg execname: the name of the executable to write.
        :arg lines: the lines of python code are appended to this list.

def _write_executable_py(self, execname, lines):
        """Writes the python executable definition for interacting with ctypes.

        :arg execname: the name of the executable to write.
        :arg lines: the lines of python code are appended to this list.
        """
        executable = self.module.executables[execname]
            
        #The 5 in the formatting indent comes from 1 space, 3 from the 'def'
        #and 1 from '('.
        inparams = []
        for p in executable.ordered_parameters:
            if "in" in p.direction:
                inparams.append(p.lname)
        pyparams = present_params(inparams, len(execname) + 5, linecont=", ")
        lines.append("def {}({}):".format(execname.lower(), pyparams))
        tab = "    "

        #Next, we want to append the docstring information from the XML tags
        #that were parsed by fortpy.
        docs = ['"""']
        summary = self._format_docstring(executable.summary)
        if len(summary) > 0:
            docs[0] += summary[0]
            docs.extend(summary[1::])
            docs.append("")
        else:
            docs[0] += "No XML documentation summary."
        for p in executable.ordered_parameters:
            doc = ":arg {}: {}; {}".format(p.name, p.strtype, p.summary)
            docs.extend(self._format_docstring(doc))

        lines.extend([tab + l for l in docs])
        lines.append(tab + '"""')

        pathstr = '{}libpath = path.join(path.dirname(__file__), "{}")'
        lines.append(pathstr.format(tab, self.libname))
        methparams = '"{}_c", "{}_c", libpath, True'.format(self.module.name, execname)
        lines.append("{}method = static_symbol({})".format(tab, methparams))
        argtypes = _py_ex_argtype(executable)
        spacing = len(tab) + 6 + 8 + 3
        lines.append("{}method.argtypes = [{}]".format(tab, present_params(argtypes, spacing,
                                                                           linecont=", ")))
        lines.append("")
        lparams = []
        _py_code_variables(lines, executable, lparams, tab)
        #This is the line that calls the shared library executable.
        methparams = present_params(lparams, len(tab) + 5, linecont=", ")
        lines.append("{}method({})\n".format(tab, methparams))

        #Now we clean up the output variables to turn them into numpy arrays etc.
        constructor = '{}result = FtypesResult("{}", "{}", "{}")'
        lines.append(constructor.format(tab, self.module.name, execname, type(executable).__name__))
        count = _py_code_clean(lines, tab, executable)
        
        if count > 0:
            lines.append("")
            lines.append("{}return result\n".format(tab))
        else:
            lines.append("{}return None\n".format(tab))