[Python-Dev] ANNOUNCE: CapPython, an object-capability subset of Python

Terry Reedy tjreedy at udel.edu
Thu Sep 18 22:33:23 CEST 2008

Mark Seaborn wrote:
> During the past couple of months I have been working on an
> object-capability subset of Python - in other words, a restricted
> execution scheme for sandboxing Python code.  It has been influenced
> by other object-capability subset languages, such as Joe-E (a subset
> of Java [1]), Caja/Cajita (subsets of Javascript [2]) and Caperl
> (based on Perl [3]).  I'm calling it CapPython because the name
> doesn't seem to have been taken yet. :-)

No wonder ;-).  I like CapPy better, though there is a shareware screen 
capture program by that name. PyCap is taken.  CapThon is not.
> I believe it is now secure, so it seems like a good time to announce
> it here!
> The basic idea behind CapPython is to enforce encapsulation by
> restricting access to private attributes of objects.  This is achieved
> through a combination of static checking and limiting access to unsafe
> builtins and modules.
> Private attributes may only be accessed through "self" variables.
> "Self" variables are defined as being the first arguments of functions
> defined inside class definitions, with a few restrictions intended to
> prevent these functions from escaping without being safely wrapped.

What about functions defined outside class definitions and then attached 
as an attribute.  Prevented?

> Private attribute names are those starting with "_".   Additionally,
> "im_self", "im_func" and some other special cases are treated as
> private attributes.

In 3.0, unbound methods are gone and im_self and im_func are __self__ 
and __func__ attributes of method objects.

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