diff --git a/.github/workflows/python-package.yml b/.github/workflows/python-package.yml
index fc7a708..5ffa075 100644
--- a/.github/workflows/python-package.yml
+++ b/.github/workflows/python-package.yml
@@ -56,4 +56,4 @@ jobs:
         ./scripts/flake8.sh
     - name: Test with pytest
       run: |
-        pytest
\ No newline at end of file
+        pytest
diff --git a/examples/family_tree.py b/examples/family_tree.py
index d8b956d..fa2293d 100644
--- a/examples/family_tree.py
+++ b/examples/family_tree.py
@@ -5,7 +5,7 @@
 #
 __author__ = "chenxm"
 
-from treelib import Tree, Node
+from treelib import Tree
 
 
 def create_family_tree():
diff --git a/examples/folder_tree.py b/examples/folder_tree.py
index 0e942e8..695d6ef 100644
--- a/examples/folder_tree.py
+++ b/examples/folder_tree.py
@@ -105,7 +105,7 @@ def get_parentid(current_depth, root, dir):
     search_string = os.path.join(root, dir)
     pos2 = search_string.rfind("/")
     pos1 = search_string.rfind("/", 0, pos2)
-    parent_dir = search_string[pos1 + 1 : pos2]
+    parent_dir = search_string[pos1 + 1 : pos2]  # noqa: E203
     parentid = (
         str(current_depth - 1)
         + "_"
@@ -130,10 +130,8 @@ def crawler():
     global FILECOUNT
 
     for root, dirs, files in os.walk(rootPath):
-
         # +++ DIRECTORIES +++
         for dir in dirs:
-
             # calculating current depth
             current_depth = os.path.join(root, dir).count("/") - start_depth
 
@@ -155,7 +153,6 @@ def crawler():
 
         # +++ FILES +++
         for filename in fnmatch.filter(files, pattern):
-
             if dir in folder_blacklist:
                 continue
 
diff --git a/examples/recursive_dirtree_generator.py b/examples/recursive_dirtree_generator.py
index 776c193..986cd1a 100644
--- a/examples/recursive_dirtree_generator.py
+++ b/examples/recursive_dirtree_generator.py
@@ -11,12 +11,21 @@
 import random
 import hashlib
 from string import digits, letters
+import sys
+
 
 MAX_FILES_PER_DIR = 10
 
 
+def range2(stop):
+    if sys.version_info[0] < 3:
+        return xrange(stop)  # noqa: F821
+    else:
+        return range(stop)
+
+
 def get_random_string(length):
-    return "".join(random.choice(digits + letters) for _ in range(length))
+    return "".join(random.choice(digits + letters) for _ in range2(length))
 
 
 def build_recursive_tree(tree, base, depth, width):
@@ -32,7 +41,7 @@ def build_recursive_tree(tree, base, depth, width):
     """
     if depth >= 0:
         depth -= 1
-        for i in xrange(width):
+        for i in range2(width):
             directory = Directory()
             tree.create_node(
                 "{0}".format(directory.name),
@@ -52,7 +61,7 @@ class Directory(object):
     def __init__(self):
         self._name = get_random_string(64)
         self._files = [
-            File() for _ in xrange(MAX_FILES_PER_DIR)
+            File() for _ in range2(MAX_FILES_PER_DIR)
         ]  # Each directory contains 1000 files
 
     @property
diff --git a/scripts/flake8.sh b/scripts/flake8.sh
index 9e05f3e..b93226d 100755
--- a/scripts/flake8.sh
+++ b/scripts/flake8.sh
@@ -1,3 +1,3 @@
 #!/bin/bash
 flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
-flake8 . --count --exit-zero --max-line-length=127 --statistics
+flake8 . --count --max-line-length=127 --exclude docs/source/conf.py --statistics
diff --git a/setup.py b/setup.py
index 559a1a7..910e467 100644
--- a/setup.py
+++ b/setup.py
@@ -1,6 +1,6 @@
 from setuptools import setup
 
-__version__ = "1.6.3"
+__version__ = "1.6.4"
 
 
 setup(
diff --git a/tests/test_plugins.py b/tests/test_plugins.py
index 05fde9d..0f04fbf 100644
--- a/tests/test_plugins.py
+++ b/tests/test_plugins.py
@@ -7,7 +7,7 @@
 import unittest
 
 from treelib import Tree
-from treelib.plugins import *
+from treelib.plugins import export_to_dot
 
 
 class DotExportCase(unittest.TestCase):
diff --git a/tests/test_tree.py b/tests/test_tree.py
index 308b6c5..b0cd934 100644
--- a/tests/test_tree.py
+++ b/tests/test_tree.py
@@ -6,10 +6,6 @@
 
 import os
 
-try:
-    from StringIO import StringIO as BytesIO
-except ImportError:
-    from io import BytesIO
 import unittest
 from treelib import Tree, Node
 from treelib.tree import NodeIDAbsentError, LoopError
@@ -304,7 +300,6 @@ def test_paste_tree(self):
         )
 
     def test_merge(self):
-
         # merge on empty initial tree
         t1 = Tree(identifier="t1")
         t2 = self.get_t2()
@@ -381,7 +376,6 @@ def test_merge(self):
         )
 
     def test_paste(self):
-
         # paste under root
         t1 = self.get_t1()
         t2 = self.get_t2()
@@ -545,7 +539,7 @@ def test_print_backend(self):
 
     def test_show(self):
         if sys.version_info[0] < 3:
-            reload(sys)
+            reload(sys)  # noqa: F821
             sys.setdefaultencoding("utf-8")
         sys.stdout = open(os.devnull, "w")  # stops from printing to console
 
diff --git a/treelib/__init__.py b/treelib/__init__.py
index d7e6ec7..4902412 100644
--- a/treelib/__init__.py
+++ b/treelib/__init__.py
@@ -35,5 +35,5 @@
    >>> from __future__ import unicode_literals
 """
 
-from .tree import Tree
-from .node import Node
+from .tree import Tree  # noqa: F401
+from .node import Node  # noqa: F401
diff --git a/treelib/tree.py b/treelib/tree.py
index 285b8c0..f3101ca 100644
--- a/treelib/tree.py
+++ b/treelib/tree.py
@@ -881,7 +881,9 @@ def show(
         former two use the same backend to generate a string of tree structure in a
         text graph.
 
-        * Version >= 1.2.7a*: you can also specify the ``line_type`` parameter, such as 'ascii' (default), 'ascii-ex', 'ascii-exr', 'ascii-em', 'ascii-emv', 'ascii-emh') to the change graphical form.
+        * Version >= 1.2.7a*: you can also specify the ``line_type`` parameter,
+          such as 'ascii' (default), 'ascii-ex', 'ascii-exr', 'ascii-em', 'ascii-emv',
+          'ascii-emh') to the change graphical form.
 
         :param nid: the reference node to start expanding.
         :param level: the node level in the tree (root as level 0).
@@ -959,7 +961,7 @@ def size(self, level=None):
                         if self.level(node.identifier) == level
                     ]
                 )
-            except:
+            except Exception:
                 raise TypeError(
                     "level should be an integer instead of '%s'" % type(level)
                 )
@@ -1063,21 +1065,33 @@ def to_json(self, with_data=False, sort=True, reverse=False):
         """To format the tree in JSON format."""
         return json.dumps(self.to_dict(with_data=with_data, sort=sort, reverse=reverse))
 
-    def to_graphviz(self, filename=None, shape="circle", graph="digraph",
-                    filter=None, key=None, reverse=False, sorting=True):
+    def to_graphviz(
+        self,
+        filename=None,
+        shape="circle",
+        graph="digraph",
+        filter=None,
+        key=None,
+        reverse=False,
+        sorting=True,
+    ):
         """Exports the tree in the dot format of the graphviz software"""
         nodes, connections = [], []
         if self.nodes:
-
-            for n in self.expand_tree(mode=self.WIDTH, filter=filter, key=key,
-                                      reverse=reverse, sorting=sorting):
+            for n in self.expand_tree(
+                mode=self.WIDTH,
+                filter=filter,
+                key=key,
+                reverse=reverse,
+                sorting=sorting,
+            ):
                 nid = self[n].identifier
                 state = '"{0}" [label="{1}", shape={2}]'.format(nid, self[n].tag, shape)
                 nodes.append(state)
 
                 for c in self.children(nid):
                     cid = c.identifier
-                    edge = '->' if graph == 'digraph' else '--'
+                    edge = "->" if graph == "digraph" else "--"
                     connections.append(('"{0}" ' + edge + ' "{1}"').format(nid, cid))
 
         # write nodes and connections to dot format