boost::python how to pass data to construct an object in python

I am using boost::python to call some python code from C++.

std::string input = "{\"x\":1}";
auto objectpath = boost::python::import("objectpath");
auto Tree = objectpath.attr("Tree")(input);
auto tree = Tree.attr("execute")("$..x");

In python the corresponding execution is (objectpath):

$ python
>>> from objectpath import *
>>> tree=Tree({"a":1})
>>> tree.execute("$.a")
1

But in C++ I am getting an error:

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/objectpath/core/interpreter.py", line 608, in execute
    ret=exe(tree)
  File "/usr/local/lib/python2.7/dist-packages/objectpath/core/interpreter.py", line 258, in exe
    fst=exe(fst)
  File "/usr/local/lib/python2.7/dist-packages/objectpath/core/interpreter.py", line 246, in exe
    return self.data
AttributeError: 'Tree' object has no attribute 'data'
Python error, but failed to get the reason via PyErr

How can I fix my C++ problem?