def _naive_sig(param_names):
    return inspect.Signature([
        inspect.Parameter(name, inspect.Parameter.POSITIONAL_OR_KEYWORD)
        for name in param_names])


class Table:
    def __init__(self, name, keys, columns):
        if not set(keys).issubset(set(columns)):
            raise ValueError('%r is not a subset of %r' %
                             (keys, columns))
        if 'default' in keys:
            raise ValueError('"default" is not a valid key name.')
        self._keys = keys
        self._columns = columns
        self._sig_add = _naive_sig(columns)
        self._sig_get = _naive_sig(keys)
        self.Keys = collections.namedtuple(name + 'Keys', keys)
        self.Record = collections.namedtuple(name + 'Record', columns)
        self._data = {}

    def _make_key(self, record):
        return self.Keys(*(getattr(record, col) for col in self._keys))

    def _make_record(self, args, kwargs):
        bound_args = self._sig_add.bind(*args, **kwargs)
        return self.Record(**bound_args.arguments)

    def add(self, *args, **kwargs):
        record = self._make_record(args, kwargs)
        key = self._make_key(record)
        if key in self._data:
            raise DuplicateKey(key)
        self._data[key] = record

    def replace(self, *args, **kwargs):
        record = self._make_record(args, kwargs)
        key = self._make_key(record)
        self._data[key] = record

    def contains(self, *args, **kwargs):
        bound_args = self._sig_get.bind(*args, **kwargs)
        key = self.Keys(**bound_args.arguments)
        return key in self._data

    def get(self, *args, default=_TABLE_GET_NO_DEFAULT, **kwargs):
        bound_args = self._sig_get.bind(*args, **kwargs)
        key = self.Keys(**bound_args.arguments)
        if default is _TABLE_GET_NO_DEFAULT:
            return self._data[key]
        else:
            return self._data.get(key, default)

    def get_dict(self, *args, **kwargs):
        val = self.get(*args, **kwargs)
        if val is None:
            return None
        return dict(zip(self._columns, map(copy.deepcopy, val)))






class Storage:
    def __init__(self):
        # ...

        self._directories = Table('Directories', ['id'], ['id', 'entries'])
        self._revisions = Table('Revisions', ['id'],
                                ['id', 'message', 'author', 'date',
                                 'committer', 'committer_date', 'parents',
                                 'type', 'directory', 'metadata',
                                 'synthetic'])
        self._releases = Table('Releases', ['id'],
                               ['id', 'name', 'author', 'date', 'target',
                                'target_type', 'message', 'synthetic'])
        self._snapshots = Table('Snapshots', ['id'],
                                ['id', 'origin', 'visit', 'branches',
                                 'sorted_branch_names'])
        self._origins = Table('Origins', ['type', 'url'],
                              ['type', 'url'])