de>def(init_expr)</code></a>, below, nor is
    it passed explicitly by users when Python instances of <code>T</code> are
    created. This idiom allows C++ virtual functions which will be overridden
    in Python to access the Python object so the Python method can be
    invoked. Boost.Python automatically registers additional converters which
    allow wrapped instances of <code>T</code> to be passed to wrapped C++
    functions expecting <code>HeldType</code> arguments.</li>

    <li>Because Boost.Pytho