diff --git a/docs/source/conf.py b/docs/source/conf.py
index b16525d..11cb707 100644
--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@@ -16,7 +16,7 @@
 import sys
 sys.path.insert(0, os.path.abspath('../..'))
 
-from treelib import __version__
+from setup import __version__
 
 # -- Project information -----------------------------------------------------
 
diff --git a/setup.py b/setup.py
index 82c2c7d..a59e128 100644
--- a/setup.py
+++ b/setup.py
@@ -1,6 +1,7 @@
 from setuptools import setup
 
-from treelib import __version__
+__version__ = '1.6.2'
+
 
 setup(
     name="treelib",
diff --git a/treelib/__init__.py b/treelib/__init__.py
index e4fef19..d7e6ec7 100644
--- a/treelib/__init__.py
+++ b/treelib/__init__.py
@@ -34,7 +34,6 @@
 
    >>> from __future__ import unicode_literals
 """
-__version__ = '1.6.1'
 
 from .tree import Tree
 from .node import Node
diff --git a/treelib/tree.py b/treelib/tree.py
index 326d887..6ee18c9 100644
--- a/treelib/tree.py
+++ b/treelib/tree.py
@@ -36,9 +36,9 @@
 
 import codecs
 import json
-import sys
 import uuid
 from copy import deepcopy
+from future.utils import python_2_unicode_compatible, iteritems
 
 try:
     from StringIO import StringIO
@@ -51,26 +51,6 @@
 __author__ = 'chenxm'
 
 
-def python_2_unicode_compatible(klass):
-    """
-    (slightly modified from: http://django.readthedocs.org/en/latest/_modules/django/utils/encoding.html)
-
-    A decorator that defines __unicode__ and __str__ methods under Python 2.
-    Under Python 3 it does nothing.
-
-    To support Python 2 and 3 with a single code base, define a __str__ method
-    returning text and apply this decorator to the class.
-    """
-    if sys.version_info[0] == 2:
-        if '__str__' not in klass.__dict__:
-            raise ValueError("@python_2_unicode_compatible cannot be applied "
-                             "to %s because it doesn't define __str__()." %
-                             klass.__name__)
-        klass.__unicode__ = klass.__str__
-        klass.__str__ = lambda self: self.__unicode__().encode('utf-8')
-    return klass
-
-
 @python_2_unicode_compatible
 class Tree(object):
     """Tree objects are made of Node(s) stored in _nodes dictionary."""
@@ -102,7 +82,7 @@ def __init__(self, tree=None, deep=False, node_class=None, identifier=None):
 
         if tree is not None:
             self.root = tree.root
-            for nid, node in tree.nodes.items():
+            for nid, node in iteritems(tree.nodes):
                 new_node = deepcopy(node) if deep else node
                 self._nodes[nid] = new_node
                 if tree.identifier != self._identifier:
@@ -154,10 +134,6 @@ def __len__(self):
         """Return len(_nodes)"""
         return len(self._nodes)
 
-    def __setitem__(self, key, item):
-        """Set _nodes[key]"""
-        self._nodes.update({key: item})
-
     def __str__(self):
         self._reader = ""
 
@@ -240,20 +216,17 @@ def filter_(node):
         return self.__get_iter(nid, level, filter_, key, reverse, dt, [])
 
     def __get_iter(self, nid, level, filter_, key, reverse, dt, is_last):
-        dt_vline, dt_line_box, dt_line_cor = dt
-
-        nid = self.root if (nid is None) else nid
-        if not self.contains(nid):
-            raise NodeIDAbsentError("Node '%s' is not in the tree" % nid)
+        dt_vertical_line, dt_line_box, dt_line_corner = dt
 
+        nid = self.root if nid is None else nid
         node = self[nid]
 
         if level == self.ROOT:
             yield "", node
         else:
-            leading = ''.join(map(lambda x: dt_vline + ' ' * 3
+            leading = ''.join(map(lambda x: dt_vertical_line + ' ' * 3
             if not x else ' ' * 4, is_last[0:-1]))
-            lasting = dt_line_cor if is_last[-1] else dt_line_box
+            lasting = dt_line_corner if is_last[-1] else dt_line_box
             yield leading + lasting, node
 
         if filter_(node) and node.expanded:
@@ -662,7 +635,7 @@ def paste(self, nid, new_tree, deep=False):
         if set_joint:
             raise ValueError('Duplicated nodes %s exists.' % list(map(text, set_joint)))
 
-        for cid, node in new_tree.nodes.items():
+        for cid, node in iteritems(new_tree.nodes):
             if deep:
                 node = deepcopy(new_tree[node])
             self._nodes.update({cid: node})
@@ -930,7 +903,7 @@ def update_node(self, nid, **attrs):
         :return: None
         """
         cn = self[nid]
-        for attr, val in attrs.items():
+        for attr, val in iteritems(attrs):
             if attr == 'identifier':
                 # Updating node id meets following contraints:
                 # * Update node identifier property