Raymond Hettinger wrote: > 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.
I just read the recipe, and it looks good to me except for the len() and iter() implementations: >>> a = Context() >>> a["x"] = 1 >>> b = a.new_child() >>> b["x"] = 2 >>> len(b) 2 >>> b.keys() ['x', 'x'] I would have expected equal keys to occur/count just once. Peter -- http://mail.python.org/mailman/listinfo/python-list
