Boost::python readonly access to nonconstant class members
Hi, is it possible to have python have readonly access to class members that are non constant? typedef map<string,string> 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. What can I do to prevent python from overwriting links? Thanks, Ralf
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<string,string> 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. 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.
What can I do to prevent python from overwriting links?
You could always do the equivalent of del DataSet.__dict__["__setitem__"] to disable it. -- Dave Abrahams BoostPro Computing http://www.boostpro.com
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<string,string> 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
At Wed, 22 Dec 2010 09:42:42 +0100, Ralf Goertz wrote:
Dave Abrahams wrote:
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...
Well, maybe the doc should be clearer, on this topic, but def_readonly makes the *attribute* read-only or read-write. Python has no distinction between const and non-const; it only has mutable and immutable objects. The map you've exposed is mutable.
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?
map_indexing_suite is indeed the thing that supplies the ability to do __setitem__ on Links. You could always wrap Links "manually" and expose the __getitem__ method yourself (but not __setitem__).
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__.
I'm sure there's a way around this, but I'm not tricky enough to figure it out right now ;-)
Maybe I can use some policy with map_indexing_suite? Sorry for my unqualified questions but I am only starting to learn python.
A mutability policy would be a good thing to add, for sure. But we don't have one today AFAIK. -- Dave Abrahams BoostPro Computing http://www.boostpro.com
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