Re: [Boost-users] Boost::python readonly access to nonconstant class members

Dave Abrahams wrote:

At Tue, 21 Dec 2010 15:47:38 +0100, Ralf Goertz wrote:

...

Hi,

is it possible to have python have readonly access to class members that are non constant?

typedef map Links;

class DataSet { Links links; }

BOOST_PYTHON_MODULE(_boosttest) { class_<Links>("Links") .def(map_indexing_suite<Links>());

class_<DataSet>("DataSet") .def_readonly("links",&DataSet::links); }

Although DataSet.links is defined readonly I can change it from within python.

If you mean that you can do

d = DataSet() d.links = 1

that would be a Boost.Python bug.

No I don't mean that and indeed that does not work: Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: can't set attribute

If instead you mean that you can do

d = DataSet() d.links["foo"] = "bar"

that would be unsurprising (to me), intended behavior; that's just how the Python object model works.

I know this would be intended if I had used def_readwrite("links",...) above. At least that's how I read http://www.boost.org/doc/libs/1_45_0/libs/python/doc/tutorial/doc/html/pytho... The only differences there to my case are that I can't have links to be a constant member of DataSet and that I am using map_indexing_suite. Is the latter the problem maybe?

...

What can I do to prevent python from overwriting links?

You could always do the equivalent of

del DataSet.__dict__["__setitem__"]

to disable it.

I have tried del Links.__dict__["__setitem__"] as I am concerned with the member variable of type Links but that gave the error TypeError: 'dictproxy' object does not support item deletion I guess map_indexing_suite creates this dictproxy which doesn't allow me to delete __setitem__. Maybe I can use some policy with map_indexing_suite? Sorry for my unqualified questions but I am only starting to learn python. Ralf