A sketch of a symbol table

symboltable.py

import collections
import contextlib
import itertools

class SymbolTable(collections.MutableMapping):
    __slots__ = ('_content')
    def __init__(self):
        self._content = []
    def _top_dict(self):
        if not self._content:
            self._content.push({})
        return self._content[-1]

    # Getters and setters
    def __getitem__(self, key):
        for d in reversed(self._content): # start with the last one pushed
            if key in d:
                return d[key]
        else:
            raise KeyError("Symbol %s not found" % repr(key))
    def __setitem__(self, key, value):
        d = self._top_dict()
        if key in d: # change d to self to not allow dynamic overrides
            raise KeyError("Can't redefine key %s" % repr(key))
        d[key] = value


    def __iter__(self, key, value):
        seen_keys = set()
        for d in reversed(self._content):
            for k in d:
                if k not in seen_keys:
                    seen_keys.add(k)
                    yield k

    def __len__(self):
        return itertools.count(self)

    
    # Context management; use like
    # with table.scope():
    #     # ... blah blah blah
    @contextlib.contextmanager
    def scope(self):
        self._content.push(dict())
        yield
        self._content.pop()