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I'm implementing a backtracking packrat parser. The inspiration for this is I'd like to have a better idea about how hygienic macros would work in an algol-like language (as apposed to the syntax free lisp dialects you normally find them in). Because of this, different passes through the input might see different grammars, so cached parse results are invalid, unless I also store the current version of the grammar along with the cached parse results. (A consequence of this use of key-value collections is that they should be immutable, but I don't intend to expose the interface to allow them to be changed, so either mutable or immutable collections are fine.) The problem is that Python dicts cannot appear as keys to other dicts. Even using a tuple (as I'd be doing anyways) doesn't help. >>> cache = {} >>> rule = {"foo": "bar"} >>> cache[(rule, "baz")] = "quux" Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'dict' >>> I guess it has to be tuples all the way down. Now the Python standard library provides approximately what I'd need, collections.namedtuple has a very different syntax, but can be used as a key. Continuing from above session: >>> from collections import namedtuple >>> Rule = namedtuple("Rule", rule.keys()) >>> cache[(Rule(**rule), "baz")] = "quux" >>> cache {(Rule(foo='bar'), 'baz'): 'quux'} Ok. But I have to make a class for each possible combination of keys in the rule I would want to use, which isn't so bad, because each parse rule knows exactly what parameters it uses, so that class can be defined at the same time as the function that parses the rule. An additional problem with namedtuple s is that they are strictly posit

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