@@ -128,13 +128,14 @@ class GroupNode(Node):
         store = None
         hash_list = []
         stack = [self]
+        alldfs = True
         while stack:
             node = stack.pop()
             if isinstance(node, GroupNode):
                 stack.extend(child for _, child in sorted(node._items(), reverse=True))
             else:
                 if not isinstance(node._node, core.TableNode):
-                    raise ValueError("Group contains non-dataframe nodes")
+                    alldfs = False
                 if not node._node.hashes:
                     msg = "Can only merge built dataframes. Build this package and try again."
                     raise NotImplementedError(msg)
"""
Nodes that represent the data in a Quilt package.
"""
import copy
import os

import pandas as pd
from six import iteritems, string_types

from .tools import core
from .tools.const import SYSTEM_METADATA
from .tools.util import is_nodename


class Node(object):
    """
    Abstract class that represents a group or a leaf node in a package.
    """
    def __init__(self, meta):
        # Can't instantiate it directly
        assert self.__class__ != Node.__class__
        assert meta is not None
        self._meta = meta

    def _class_repr(self):
        """Only exists to make it easier for subclasses to customize `__repr__`."""
        return "<%s>" % self.__class__.__name__

    def __repr__(self):
        return self._class_repr()

    def __call__(self, asa=None):
        return self._data(asa)

    def _data(self, asa=None):
        raise NotImplementedError


class DataNode(Node):
    """
    Represents a dataframe or a file. Allows accessing the contents using `()`.
    """
    def __init__(self, package, node, data, meta):
        super(DataNode, self).__init__(meta)

        self._package = package
        self._node = node
        self.__cached_data = data

    def _data(self, asa=None):
        """
        Returns the contents of the node: a dataframe or a file path, or passes
        the node and its contents to a callable.        
        """
        if asa is not None:
            if self._package is None or not self._node.hashes:
                msg = "Can only use asa functions with built dataframes."
                " Build this package and try again."
                raise ValueError(msg)
            store = self._package.get_store()
            return asa(self, [store.object_path(obj) for obj in self._node.hashes])
        else:
            if self.__cached_data is None:
                # TODO(dima): Temporary code.
                store = self._package.get_store()
                if isinstance(self._node, core.TableNode):
                    self.__cached_data = store.load_dataframe(self._node.hashes)
                elif isinstance(self._node, core.FileNode):
                    self.__cached_data = store.get_file(self._node.hashes)
                else:
                    assert False
            return self.__cached_data


class GroupNode(Node):
    """
    Represents a group in a package. Allows accessing child objects using the dot notation.
    Warning: calling _data() on a large dataset may exceed local memory capacity in Python (Only
    supported for Parquet packages).
    """
    def __init__(self, meta):
        super(GroupNode, self).__init__(meta)

    def __setattr__(self, name, value):
        if name.startswith('_') or isinstance(value, Node):
            super(Node, self).__setattr__(name, value)
        else:
            raise AttributeError("{val} is not a valid package node".format(val=value))

    def __repr__(self):
        pinfo = super(GroupNode, self).__repr__()
        group_info = '\n'.join(name + '/' for name in sorted(self._group_keys()))
        if group_info:
            group_info += '\n'
        data_info = '\n'.join(sorted(self._data_keys()))
        return '%s\n%s%s' % (pinfo, group_info, data_info)

    def _items(self):
        return ((name, child) for name, child in iteritems(self.__dict__)
                if not name.startswith('_'))

    def _data_keys(self):
        """
        every child key referencing a dataframe
        """
        return [name for name, child in self._items() if not isinstance(child, GroupNode)]

    def _group_keys(self):
        """
        every child key referencing a group that is not a dataframe
        """
        return [name for name, child in self._items() if isinstance(child, GroupNode)]

    def _keys(self):
        """
        keys directly accessible on this object via getattr or .
        """
        return [name for name in self.__dict__ if not name.startswith('_')]

    def _add_group(self, groupname):
        child = GroupNode({})
        setattr(self, groupname, child)

    def _data(self, asa=None):
        """
        Merges all child dataframes. Only works for dataframes stored on disk - not in memory.
        """
        store = None
        hash_list = []
        stack = [self]
        while stack:
            node = stack.pop()
            if isinstance(node, GroupNode):
                stack.extend(child for _, child in sorted(node._items(), reverse=True))
            else:
                if not isinstance(node._node, core.TableNode):
                    raise ValueError("Group contains non-dataframe nodes")
                if not node._node.hashes:
                    msg = "Can only merge built dataframes. Build this package and try again."
                    raise NotImplementedError(msg)
                node_store = node._package.get_store()
                if store is None:
                    store = node_store
                elif node_store is not store:
                    raise NotImplementedError("Can only merge dataframes from the same store")
                hash_list += node._node.hashes

        if asa is None:
            if not hash_list:
                return None
            return store.load_dataframe(hash_list)
        else:
            if hash_list:
                assert store is not None
                return asa(self, [store.object_path(obj) for obj in hash_list])
            else:
                return asa(self, [])


def _create_filter_func(filter_dict):
    filter_name = filter_dict.pop('name', None)
    if filter_name is not None and not isinstance(filter_name, string_types):
        raise ValueError("Invalid 'name': %r" % filter_name)

    filter_meta = filter_dict.pop('meta', None)
    if filter_meta is not None and not isinstance(filter_meta, dict):
        raise ValueError("Invalid 'meta': %r" % filter_meta)

    if filter_dict:
        raise ValueError("Unexpected data in the filter: %r; only 'name' and 'meta' are supported" % filter_dict)

    def helper(value, expected):
        if isinstance(expected, dict):
            if isinstance(value, dict):
                for expected_key, expected_value in iteritems(expected):
                    if not helper(value.get(expected_key), expected_value):
                        return False
                return True
            else:
                return False
        else:
            return value == expected

    def func(node, name):
        if filter_name is not None and filter_name != name:
            return False
        if filter_meta is not None and not helper(node._meta, filter_meta):
            return False
        return True

    return func

class PackageNode(GroupNode):
    """
    Represents a package.
    """
    def __init__(self, package, meta):
        super(PackageNode, self).__init__(meta)
        self._package = package

    def _class_repr(self):
        finfo = self._package.get_path() if self._package is not None else ''
        return "<%s %r>" % (self.__class__.__name__, finfo)

    def _set(self, path, value, build_dir=''):
        """Create and set a node by path

        This creates a node from a filename or pandas DataFrame.

        If `value` is a filename, it must be relative to `build_dir`.
            `value` is stored as the export path.

        `build_dir` defaults to the current directory, but may be any
            arbitrary directory path, including an absolute path.

        Example:
            # Set `pkg.graph_image` to the data in '/home/user/bin/graph.png'.
            # If exported, it would export to '<export_dir>/bin/graph.png'
            `pkg._set(['graph_image'], 'bin/fizz.bin', '/home/user')`

        :param path:  Path list -- I.e. ['examples', 'new_node']
        :param value:  Pandas dataframe, or a filename relative to build_dir
        :param build_dir:  Directory containing `value` if value is a filename.
        """
        assert isinstance(path, list) and len(path) > 0

        if isinstance(value, pd.DataFrame):
            metadata = {}
            core_node = core.TableNode(hashes=[], format=core.PackageFormat.default.value)
        elif isinstance(value, string_types + (bytes,)):
            # bytes -> string for consistency when retrieving metadata
            value = value.decode() if isinstance(value, bytes) else value
            if os.path.isabs(value):
                raise ValueError("Invalid path: expected a relative path, but received {!r}".format(value))
            # Security: filepath does not and should not retain the build_dir's location!
            metadata = {SYSTEM_METADATA: {'filepath': value, 'transform': 'id'}}
            core_node = core.FileNode(hashes=[])
            if build_dir:
                value = os.path.join(build_dir, value)
        else:
            accepted_types = tuple(set((pd.DataFrame, bytes) + string_types))
            raise TypeError("Bad value type: Expected instance of any type {!r}, but received type {!r}"
                            .format(accepted_types, type(value)), repr(value)[0:100])

        for key in path:
            if not is_nodename(key):
                raise ValueError("Invalid name for node: {}".format(key))

        node = self
        for key in path[:-1]:
            child = getattr(node, key, None)
            if not isinstance(child, GroupNode):
                child = GroupNode({})
                setattr(node, key, child)

            node = child

        key = path[-1]
        data_node = DataNode(self._package, core_node, value, metadata)
        setattr(node, key, data_node)

    def _filter(self, lambda_or_dict):
        if isinstance(lambda_or_dict, dict):
            func = _create_filter_func(lambda_or_dict)
        elif callable(lambda_or_dict):
            func = lambda_or_dict
        else:
            raise ValueError

        def _filter_node(name, node, func):
            matched = func(node, name)
            if isinstance(node, GroupNode):
                if isinstance(node, PackageNode):
                    filtered = PackageNode(None, copy.deepcopy(node._meta))
                else:
                    filtered = GroupNode(copy.deepcopy(node._meta))
                for child_name, child_node in node._items():
                    # If the group itself matched, then match all children by using a True filter.
                    child_func = (lambda *args: True) if matched else func
                    filtered_child = _filter_node(child_name, child_node, child_func)
                    if filtered_child is not None:
                        setattr(filtered, child_name, filtered_child)

                # Return the group if:
                # 1) It has children, or
                # 2) Group itself matched the filter, or
                # 3) It's the package itself.
                if matched or next(filtered._items(), None) or node == self:
                    return filtered
            else:
                if matched:
                    return node
            return None

        return _filter_node('', self, func)

You probably need to check for `node._node is None` now, too (same as in the previous PR)