Add example on how to call Python function in C++.

Author: luator

CMakeLists.txt

@@ -40,6 +40,17 @@ target_link_libraries(mycpplib
 )
 
 
+add_executable(use_pylib
+  src/use_pylib.cpp
+)
+
+target_link_libraries(use_pylib
+  ${catkin_LIBRARIES}
+  ${Boost_LIBRARIES}
+  ${PYTHON_LIBRARIES}
+)
+
+
 #############
 ## Install ##
 #############

package.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <package>
   <name>boost_python_catkin_example</name>
-  <version>1.0.0</version>
+  <version>1.1.0</version>
   <description>
     A minimal example package on how to use Boost::Python with catkin.
   </description>

src/boost_python_catkin_example/mypylib.py

@@ -0,0 +1,18 @@
+"""Some simple example module that is going to be used in C++."""
+
+
+def hello():
+    """Return a hello message."""
+    return "Hello C++. I'm Python, looking forward to work with you!"
+
+
+class SomeClass:
+
+    def __init__(self):
+        self.value = None
+
+    def set_value(self, value):
+        self.value = value
+
+    def get_value(self):
+        return self.value

src/use_pylib.cpp

@@ -0,0 +1,67 @@
+/**
+ * \file
+ * \brief Simple example on how to call a Python function from C++ code.
+ *
+ * Useful resources:
+ *  - Actually the boost python documentation starts to get a bit better:
+ *    http://www.boost.org/doc/libs/1_61_0/libs/python/doc/html/tutorial/tutorial/embedding.html
+ *  - A bit outdated but contains a function to extract traceback from
+ *    exceptions: https://wiki.python.org/moin/boost.python/EmbeddingPython
+ */
+#include <iostream>
+#include <string>
+#include <boost/python.hpp>
+
+using namespace boost::python;
+
+int main( int argc, char ** argv ) {
+    // It is important to initialize the Python interpreter before running any
+    // Python code.
+    Py_Initialize();
+
+    try
+    {
+        // based on the example from
+        // http://www.boost.org/doc/libs/1_46_0/libs/python/doc/v2/exec.html
+
+        // Retrieve the main module.
+        object main = import("__main__");
+
+        // Retrieve the main module's namespace
+        object global(main.attr("__dict__"));
+
+        // Import my module
+        object mypylib = import("boost_python_catkin_example.mypylib");
+
+        // Create a reference to the hello function in the module.
+        object hello = mypylib.attr("hello");
+        object SomeClass = mypylib.attr("SomeClass");
+
+        // Call it.
+        std::string message = extract<std::string>(hello());
+        std::cout << message << std::endl;
+
+        object some_class_obj = SomeClass();
+        some_class_obj.attr("set_value")(42);
+        int value = extract<int>(some_class_obj.attr("get_value")());
+        std::cout << "Value: " << value << std::endl;
+    }
+    catch (const error_already_set &)
+    {
+        // When the Python code raises an error, this will result in a
+        // boost::python::error_already_set exception in C++.  Unfortunately
+        // this exception does not contain any information about the nature of
+        // the error, so we need to interface with the Python C API to get more
+        // information.
+        PyErr_Print();
+
+        // To handle based on exception type:
+        //if (PyErr_ExceptionMatches(PyExc_ZeroDivisionError))
+        //{
+        //    // handle ZeroDivisionError specially
+        //}
+
+        return 1;
+    }
+    return 0;
+}