An essential part of integration of Scheme (as implemented in Guile) and Python is allowing Python code to call back code implemented in Scheme. It is also desirable to be able to access data and invoke methods on otherwise-opaque objects created and managed in Scheme. The specifics of opaque object access should also be independent of the specific object system being used in the Scheme-written part of the application.
When implementing in PyGuile the proxying scheme, in which PyObjects serve as proxies for SCMs, the following need to be taken into account:
- Data type conversion
- Retention of references as a protection against garbage collection
Data type conversions
A Python callback can be expected to receive positional and keyword arguments, and return a result of any type. Therefore, templates (possibly trivial) for converting between PyObjects (Python data types) and SCMs (Guile data types) need to be associated with each callback.
In the case of objects, we need to associate, with each attribute getter, a template for converting the value from SCM into PyObject. With each attribute setter, a template for converting the value from PyObject into SCM needs to be associated. With each method, which can be invoked on the object, minimum two templates are needed. Three templates should be provided, in case the object needs to be manipulated by an interface, which expects both positional and keyword arguments in the object’s methods.
All the templates needed to work with a SCM (as a callback or as an object) are associated with it when it is wrapped as it is being passed from Guile to Python.
Retention of references
PyObjects, which wrap SCMs, are not expected to be seen by Guile’s garbage collector. Therefore, we need a mechanism for protecting SCMs referenced by PyObjects.
Due to efficiency considrations, Guile’s scm_permanent_object, scm_gc_protect_object or scm_gc_unprotect_object should not be used on every SCM passed to Python. The solution is to create a set object in Guile, protect it using scm_permanent_object (a single call) and then register in it all wrapped SCMs. When a wrapping PyObject’s __del__ function is invoked, one of its actions is to remove the corresponding SCM object from the set. The set will be implemented using a standard hash table, whose keys will be indexes and the data – the SCMs themselves.