Recipe 6.4. Chaining Dictionary LookupsCredit: Raymond Hettinger ProblemYou have several mappings (usually dicts) and want to look things up in them in a chained way (try the first one; if the key is not there, then try the second one; and so on). Specifically, you want to make a single mapping object that "virtually merges" several others, by looking things up in them in a specified priority order, so that you can conveniently pass that one object around. SolutionA mapping is a generalized, abstract version of a dictionary: a mapping provides an interface that's similar to a dictionary's, but it may use very different implementations. All dictionaries are mappings, but not vice versa. Here, you need to implement a mapping which sequentially tries delegating lookups to other mappings. A class is the right way to encapsulate this functionality: class Chainmap(object): def _ _init_ _(self, *mappings): # record the sequence of mappings into which we must look self._mappings = mappings def _ _getitem_ _(self, key): # try looking up into each mapping in sequence for mapping in self._mappings: try: return mapping[key] except KeyError: pass # `key' not found in any mapping, so raise KeyError exception raise KeyError, key def get(self, key, default=None): # return self[key] if present, otherwise `default' try: return self[key] except KeyError: return default def _ _contains_ _(self, key): # return True if `key' is present in self, otherwise False try: self[key] return True except KeyError: return False For example, you can now implement the same sequence of lookups that Python normally uses for any name: look among locals, then (if not found there) among globals, lastly (if not found yet) among built-ins: import _ _builtin_ _ pylookup = Chainmap(locals( ), globals( ), vars(_ _builtin_ _)) DiscussionChainmap relies on minimal functionality from the mappings it wraps: each of those underlying mappings must allow indexing (i.e., supply a special method _ _getitem_ _), and it must raise the standard exception KeyError when indexed with a key that the mapping does not know about. A Chainmap instance provides the same behavior, plus the handy get method covered in Recipe 4.9 and special method _ _contains_ _ (which conveniently lets you check whether some key k is present in a Chainmap instance c by just coding if k in c). Besides the obvious and sensible limitation of being "read-only", this Chainmap class has othersessentially, it is not a "full mapping" even within the read-only design choice. You can make any partial mapping into a "full mapping" by inheriting from class DictMixin (in standard library module UserDict) and supplying a few key methods (DictMixin implements the others). Here is how you could make a full (read-only) mapping from ChainMap and UserDict.DictMixin: import UserDict from sets import Set class FullChainmap(Chainmap, UserDict.DictMixin): def copy(self): return self._ _class_ _(self._mappings) def _ _iter_ _(self): seen = Set( ) for mapping in self._mappings: for key in mapping: if key not in seen: yield key seen.add(key) iterkeys = _ _iter_ _ def keys(self): return list(self) This class FullChainmap adds one requirement to the mappings it holds, besides the requirements posed by Chainmap: the mappings must be iterable. Also note that the implementation in Chainmap of methods get and _ _contains_ _ is redundant (although innocuous) once we subclass DictMixin, since DictMixin also implements those two methods (as well as many others) in terms of lower-level methods, just like Chainmap does. See Recipe 5.14 for more details about DictMixin. See AlsoRecipe 4.9; Recipe 5.14; the Library Reference and Python in a Nutshell sections on mapping types. |