I'm considering a nested mapping class for the collections module and would like to solicit feedback from people here on comp.lang.python:
http://code.activestate.com/recipes/577434-nested-contexts-a-chain-of-mapping-objects/ The class is an attempt to generalize from existing use cases (i.e. the _multimap class in string.py). I also took guidance from the symbol table analysis tools in ANTLR (see p.173 of Language Implementation Patterns by Terence Parr). Nested mappings make it easy to build chains and trees of scopes (like vocabularies in Forth or like inheritance hierarchies in Python). The basic idea is to perform look-ups into a series a dictionaries, much like Python does with locals, nested scopes, global (module level) scope and builtins. Starting with a root context which has a full dictionary API, multiple child contexts can be created. New values are stored in the child contexts, potentially masking those in a parent context (much like shadowing a Python builltin). Lookups start with the child node and work upward through the chain of parents. There is also a variant where __setitem__ and __delitem__ can mutate values in parent contexts (much like the nonlocal keyword allows in Python). The API seeks to fully emulate regular dictionaries. The only extra capabilities are: d = c.new_child() # create a subcontext e = d.new_child() # create a subsubcontext e.parent # find the next enclosing context e.root # find the outermost enclosing context f = c.new_child() # create a subcontext of c, independent of d & e I would appreciate any feedback on the API and on how well it fits with various use cases that you've found in the wild. Raymond -- http://mail.python.org/mailman/listinfo/python-list
