Boost::python readonly access to nonconstant class members

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Ralf Goertz

21 Dec 2010 21 Dec '10

2:47 p.m.

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. What can I do to prevent python from overwriting links? Thanks, Ralf

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Dave Abrahams

21 Dec 21 Dec

7:14 p.m.

New subject: Boost::python readonly access to nonconstant class members

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. 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

Ralf Goertz

22 Dec 22 Dec

8:42 a.m.

New subject: 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

Dave Abrahams

26 Dec 26 Dec

12:46 a.m.

New subject: Boost::python readonly access to nonconstant class members

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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