diff --git a/test/test_tkoutw.py b/test/test_tkoutw.py
index 917b006..0235ff8 100644
--- a/test/test_tkoutw.py
+++ b/test/test_tkoutw.py
@@ -1,11 +1,7 @@
-from io import StringIO
 from tkinter import *
-from tkinter import ttk
 import unittest
 
-from lxml import etree
-from lxml.cssselect import CSSSelector
-from tkouter.core import TkGridMgr, TkOutWidget, TkOutElement, register
+from tkouter.core import TkGridMgr, TkOutWidget, register
 from tkouter.errors import *
 from tkouter.fields import *
 from tkouter import settings
@@ -28,7 +24,9 @@
             <body>
                 <notebook name="nb">
                     <left type="labelframe" pack-fill="both">
-                        <button command="{self.test}"> {self.data_dict.button_text} </button>
+                        <button command="{self.test}">
+                            {self.data_dict.button_text}
+                        </button>
                         <entry id="0" textvariable="{self.strfield.var}" />
                         <entry id="1" textvariable="{self.boolfield.var}" />
                         <entry id="2" textvariable="{self.intfield.var}" />
@@ -37,11 +35,17 @@
                 {% if test_grid %}
                 <grid>
                     <gr id="gr0">
-                        <gd id="gd0"><button id="bt0" text="grid button 0" /></gd>
-                        <gd id="gd1" rowspan="2" columnspan="2"><button id="bt1" text="grid button 1" /></gd>
+                        <gd id="gd0">
+                            <button id="bt0" text="grid button 0" />
+                        </gd>
+                        <gd id="gd1" rowspan="2" columnspan="2">
+                            <button id="bt1" text="grid button 1" />
+                        </gd>
                     </gr>
                     <gr id="gr1">
-                        <gd id="gd2"><button id="bt2" text="grid button 2" /></gd>
+                        <gd id="gd2">
+                            <button id="bt2" text="grid button 2" />
+                        </gd>
                     </gr>
                 </grid>
                 {% endif %}
@@ -56,6 +60,7 @@
         }
     """
 
+
 @register('testwidget')
 class TestWidget(TkOutWidget):
     strfield = StringField(default='str', max_length=5)
@@ -69,6 +74,7 @@ class TestWidget(TkOutWidget):
     def test(self):
         pass
 
+
 class TestWidgetWithCss(TkOutWidget):
     strfield = StringField(default='str', max_length=5)
     boolfield = BoolField(default=True)
@@ -83,35 +89,48 @@ def test(self):
         for b in self.select('button'):
             b.config(text='change')
 
+
 class TestWidgetWithoutLayout(TkOutWidget):
     pass
 
+
 class TestTagUnRecognizedError(TkOutWidget):
     layout = """<html><hello> invalid </hello></html>"""
 
+
 class TestDataNotExistError(TkOutWidget):
-    layout = """<html><head></head><body><button command="{self.nofunc}" /></body></html>"""
+    layout = """<html><head></head><body>
+                <button command="{self.nofunc}" /></body></html>"""
+
 
 class TestTagInWrongScopeOutSide(TkOutWidget):
     layout = """<html><radiobutton /><head></head><body></body></html>"""
 
+
 class TestTagInWrongScopeHead(TkOutWidget):
     layout = """<html><head><button /></head><body></body></html>"""
 
+
 class TestTagInWrongScopeMenu(TkOutWidget):
-    layout = """<html><head><menu><title></title></menu></head><body></body></html>"""
+    layout = """<html><head><menu><title></title></menu></head>
+                <body></body></html>"""
+
 
 class TestTagInWrongScopeBody(TkOutWidget):
     layout = """<html><head></head><body><command /></body></html>"""
 
+
 class TestTagInWrongScopeGrid(TkOutWidget):
     layout = """<html><body><grid><gd></gd></grid></body></html>"""
 
+
 class TestTagInWrongScopeGr(TkOutWidget):
     layout = """<html><body><grid><gr><button /></gr></grid></body></html>"""
 
+
 class TestTagInWrongScopeGd(TkOutWidget):
-    layout = """<html><body><grid><gr><gd><gr></gr></gd></gr></grid></body></html>"""
+    layout = """<html><body><grid><gr><gd><gr>
+                </gr></gd></gr></grid></body></html>"""
 
 
 class TestTkOutWidget(unittest.TestCase):
@@ -135,7 +154,6 @@ def test_widget(self):
         left = self.select_one_element('body left')
         notebook = self.select_one_element('body > notebook')
         button = self.select_one_element('left > button')
-        command = self.select_one_element('menu > command')
         radiobutton = self.select_one_element('menu > radiobutton')
         entry_0 = self.select_one_element('left > entry#0')
         grid = self.select_one_element('body > grid')
@@ -154,7 +172,8 @@ def test_widget(self):
         self.assertTrue(head.is_scope and not title.is_scope)
         self.assertTrue(left.is_side and not body.is_side)
         self.assertTrue(grid.is_grid and gr_0.is_gr and gd_0.is_gd)
-        self.assertTrue(not grid.is_grid_element and gr_0.is_grid_element and gd_0.is_grid_element)
+        self.assertTrue(not grid.is_grid_element and gr_0.is_grid_element and
+                        gd_0.is_grid_element)
         self.assertTrue(top_menu.is_menu and sub_menu.is_menu)
         self.assertTrue(top_menu.is_top_menu)
         self.assertTrue(sub_menu.is_sub_menu)
@@ -167,20 +186,24 @@ def test_widget(self):
         self.assertTrue(gd_0.is_in_gr and not gr_0.is_in_gr)
         self.assertTrue(button_g0.is_in_gd and not gd_0.is_in_gd)
         self.assertTrue(css.can_under_head and not left.can_under_head)
-        self.assertTrue(radiobutton.can_under_menu and not button.can_under_menu)
+        self.assertTrue(radiobutton.can_under_menu and
+                        not button.can_under_menu)
         self.assertTrue(notebook.can_under_body and not title.can_under_body)
         self.assertFalse(top_menu.can_under_body or sub_menu.can_under_body)
         self.assertTrue(gr_0.can_in_grid and not grid.can_in_grid)
         self.assertTrue(gd_0.can_in_gr and not gr_0.can_in_gr)
-        self.assertTrue(button_g0.can_in_gd and not gd_0.can_in_gd and not top_menu.can_in_gd)
+        self.assertTrue(button_g0.can_in_gd and not gd_0.can_in_gd and
+                        not top_menu.can_in_gd)
         # widget checker
         self.assertTrue(title.has_no_widget_type and html.has_no_widget_type)
         self.assertTrue(button.has_widget_type and radiobutton.has_widget_type)
         self.assertTrue(button.has_widget_name and not title.has_widget_name)
         self.assertTrue(left.has_widget_cls and not title.has_widget_cls)
         self.assertTrue(button.has_options and not notebook.has_options)
-        self.assertTrue(button.has_widget and top_menu.has_widget and not css.has_widget)
-        self.assertTrue(grid.has_gridmgr and not gr_0.has_gridmgr and not gd_0.has_gridmgr)
+        self.assertTrue(button.has_widget and top_menu.has_widget and
+                        not css.has_widget)
+        self.assertTrue(grid.has_gridmgr and not gr_0.has_gridmgr and
+                        not gd_0.has_gridmgr)
         # widget attr
         self.assertIsNone(title.widget_type)
         self.assertEqual(left.widget_type, 'labelframe')
@@ -198,10 +221,13 @@ def test_widget(self):
         self.assertEqual(grid.gridmgr, gd_0.gridmgr)
         self.assertEqual(button._options['command'], self.tkoutw.test)
         self.assertEqual(button._options['text'], 'test button')
-        self.assertEqual(entry_0._options['textvariable'], self.tkoutw.__class__.__dict__['strfield'].var)
+        self.assertEqual(entry_0._options['textvariable'],
+                         self.tkoutw.__class__.__dict__['strfield'].var)
         self.assertEqual(left.pack_options['fill'], 'both')
         self.assertEqual(gd_0.grid_options, {'row': 0, 'column': 0})
-        self.assertEqual(gd_1.grid_options, {'row': 0, 'column': 1, 'rowspan': '2', 'columnspan': '2'})
+        self.assertEqual(gd_1.grid_options, {'row': 0, 'column': 1,
+                                             'rowspan': '2',
+                                             'columnspan': '2'})
         self.assertEqual(gd_2.grid_options, {'row': 1, 'column': 0})
         self.assertIsNone(title.widget)
         self.assertIsInstance(button.widget, Button)
@@ -269,4 +295,4 @@ def test_grid_mgr(self):
 
 
 if __name__ == '__main__':
-    unittest.main()
\ No newline at end of file
+    unittest.main()
diff --git a/tkouter/core.py b/tkouter/core.py
index b8cfc20..c752ef8 100644
--- a/tkouter/core.py
+++ b/tkouter/core.py
@@ -2,14 +2,13 @@
 """
 
 
-__all__ =  [
+__all__ = [
     'register',
     'TkOutWidget',
 ]
 
 
 from io import StringIO
-from html.parser import HTMLParser
 from tkinter import Frame, Menu
 from tkinter import ttk
 
@@ -24,8 +23,8 @@
 
 def register(name):
     """ function to add additional widget to BODY_WIDGETS
-    usage: 
-        register('my_widget_name')(MyWidgetClass) 
+    usage:
+        register('my_widget_name')(MyWidgetClass)
     """
     def _register(widget_cls):
         settings.WIDGETS[name] = widget_cls
@@ -42,7 +41,7 @@ def get_column(self, row, rowspan=1, colspan=1):
         rsegments = self._segments.setdefault(row, [])
         if not rsegments:
             return 0
-        lo = rsegments[0][0]
+
         hi = rsegments[-1][-1]
         for col in range(0, hi + 2):
             if self._has_space(row, col, rowspan, colspan):
@@ -115,12 +114,14 @@ def init(self, tkoutw):
 
     # top checker
     def _check_valid(self):
-        if not (self.is_html or self.is_scope or self.can_under_head or self.can_under_body):
+        if not (self.is_html or self.is_scope or self.can_under_head or
+                self.can_under_body):
             msg = 'unrecognized tag <{}>'
             raise TagUnRecognizedError(msg.format(self.tag))
 
     def _check_scope(self):
-        if not (self.is_html or self.is_scope or self.is_under_head or self.is_under_body):
+        if not (self.is_html or self.is_scope or self.is_under_head or
+                self.is_under_body):
             msg = 'tag <{}> should be under tag <head> or <body>'
             raise TagInWrongScope(msg.format(self.tag))
         elif self.is_under_head and not self.can_under_head:
@@ -151,8 +152,8 @@ def _handle_options(self, options):
         execution.
 
         Some options are special or complicated, we should pre-set their values
-        and assign to some variables then specify the variable in symbol "{" and
-        "}" as option value.
+        and assign to some variables then specify the variable in symbol
+        "{" and "}" as option value.
         """
         modified_options = {}
         for name, value in options.items():
@@ -170,7 +171,7 @@ def _handle_options(self, options):
                         elif attr in data:
                             data = data[attr]
                     modified_options[name] = data
-                except:
+                except TypeError:
                     msg = 'data "{}" does not exist'
                     raise DataNotExistError(msg.format(value))
             else:
@@ -182,7 +183,7 @@ def _parse_options(self):
             if attr in ['name', 'type', 'class', 'id']:
                 continue
             elif '-' in attr:
-                method, _, attr  = attr.partition('-')
+                method, _, attr = attr.partition('-')
                 options = self._widget_method_options.setdefault(method, {})
                 options[attr] = value
             else:
@@ -225,7 +226,7 @@ def _init_grid_options(self):
     # tag category
     @property
     def is_html(self):
-        return self.tag == 'html' or self.tag =='xml'
+        return self.tag == 'html' or self.tag == 'xml'
 
     @property
     def is_head(self):
@@ -285,7 +286,8 @@ def is_sub_menu(self):
 
     @property
     def is_notebook(self):
-        return self.has_widget_cls and issubclass(self.widget_cls, ttk.Notebook)
+        return (self.has_widget_cls and
+                issubclass(self.widget_cls, ttk.Notebook))
 
     # scope checker
     @property
@@ -318,15 +320,18 @@ def is_in_gd(self):
 
     @property
     def can_under_head(self):
-        return self.is_root_attr or self.is_link or self.is_menu or self.can_under_menu
+        return (self.is_root_attr or self.is_link or self.is_menu or
+                self.can_under_menu)
 
     @property
     def can_under_menu(self):
-        return self.tag in ['separator', 'command', 'radiobutton', 'checkbutton'] or self.is_sub_menu
+        return (self.tag in ['separator', 'command', 'radiobutton',
+                             'checkbutton'] or self.is_sub_menu)
 
     @property
     def can_under_body(self):
-        return (self.tag in self.widgets and not self.is_menu) or self.is_side or self.is_grid or self.is_grid_element
+        return ((self.tag in self.widgets and not self.is_menu) or
+                self.is_side or self.is_grid or self.is_grid_element)
 
     @property
     def can_in_grid(self):
@@ -343,7 +348,8 @@ def can_in_gd(self):
     # widget checker
     @property
     def has_no_widget_type(self):
-        return self.is_html or self.is_scope or self.is_root_attr or self.is_grid_element
+        return (self.is_html or self.is_scope or self.is_root_attr or
+                self.is_grid_element)
 
     @property
     def has_widget_type(self):
@@ -389,7 +395,8 @@ def widget_name(self):
                     self.widget_type_counter[self.widget_type] += 1
                 else:
                     self.widget_type_counter[self.widget_type] = 0
-                self._name = self.widget_type + '_' + str(self.widget_type_counter[self.widget_type])
+                self._name = (self.widget_type + '_' +
+                              str(self.widget_type_counter[self.widget_type]))
         return self._name
 
     @property
@@ -429,7 +436,8 @@ def widget(self):
         if self._widget is None:
             assert(self.parent_widget)
             if self.is_under_body and not self.is_grid_element:
-                self._widget = self.widget_cls(self.parent_widget, **self._options)
+                self._widget = self.widget_cls(self.parent_widget,
+                                               **self._options)
                 setattr(self.tkoutw, self.widget_name, self._widget)
             elif self.is_menu:
                 self._widget = self.widget_cls(self.parent_widget)
@@ -438,13 +446,16 @@ def widget(self):
 
     # core function
     def display(self):
-        if self.is_html or self.is_scope or self.is_link or self.is_grid_element:
+        if (self.is_html or self.is_scope or self.is_link or
+                self.is_grid_element):
             pass
         elif self.is_under_head:
             if self.is_sub_menu:
-                self.parent_widget.add_cascade(menu=self.widget, **self._options)
+                self.parent_widget.add_cascade(menu=self.widget,
+                                               **self._options)
             elif self.is_under_menu:
-                self.parent_widget.add(itemType=self.widget_type, **self._options)
+                self.parent_widget.add(itemType=self.widget_type,
+                                       **self._options)
             elif self.is_top_menu:
                 self.parent_widget['menu'] = self.widget
             elif self.is_root_attr:
@@ -456,14 +467,16 @@ def display(self):
             else:
                 self.widget.pack(**self.pack_options)
             if self.getparent().is_notebook:
-                self.parent_widget.add(child=self.widget, text=self.widget_name)
+                self.parent_widget.add(child=self.widget,
+                                       text=self.widget_name)
 
 
 class TkOutWidget(Frame):
     """ Design a user-defined widget with html-based layout
 
     User could define a html-based layout widget just by inheriting this class.
-    Subclass may override some class attributes to make its own widget or layout.
+    Subclass may override some class attributes
+    to make its own widget or layout.
 
     Public attributes:
     - widgets: available widgets (dictionary)
@@ -507,7 +520,6 @@ def _build(self):
         self._proxy_cache = list(self._tree.getroot().iter())
 
         # css
-        css = None
         for e in self._tree.getroot().iter():
             if e.is_css and e.get('href'):
                 self._css = env.get_template(e.get('href')).render()
@@ -526,7 +538,8 @@ def _build(self):
                 e.display()
             except TagError as err:
                 print('Error when parsing tag: ')
-                print(etree.tostring(e, pretty_print=True, encoding=str, method='html'))
+                print(etree.tostring(e, pretty_print=True, encoding=str,
+                                     method='html'))
                 raise err
 
     def _select(self, selector_str):
@@ -536,4 +549,5 @@ def _select(self, selector_str):
 
     def select(self, selector_str):
         """ use css selector string to query corresponding widgets """
-        return (e.widget for e in self._select(selector_str) if e.widget is not None)
\ No newline at end of file
+        return (e.widget for e in self._select(selector_str)
+                if e.widget is not None)
diff --git a/tkouter/errors.py b/tkouter/errors.py
index f0a76cf..0ff24d6 100644
--- a/tkouter/errors.py
+++ b/tkouter/errors.py
@@ -13,14 +13,18 @@
 class Error(Exception):
     """ Base-class for all exceptions raised by this module. """
 
+
 class TagError(Error):
     """ There is a problem in the tag of layout html. """
 
+
 class TagUnRecognizedError(TagError):
     """ There is unknown tag in tkouter layout html. """
 
+
 class DataNotExistError(TagError):
     """ can not find the data specified in tag from data_context. """
 
+
 class TagInWrongScope(TagError):
-    """ tag in wrong scope """
\ No newline at end of file
+    """ tag in wrong scope """
diff --git a/tkouter/fields.py b/tkouter/fields.py
index 75d2817..fc451e6 100644
--- a/tkouter/fields.py
+++ b/tkouter/fields.py
@@ -80,4 +80,4 @@ def __get__(self, instance, owner):
         return self.var.get()
 
     def __set__(self, instance, value):
-        self.var.set(value)
\ No newline at end of file
+        self.var.set(value)
diff --git a/tkouter/settings.py b/tkouter/settings.py
index 6d34c33..ea0d725 100644
--- a/tkouter/settings.py
+++ b/tkouter/settings.py
@@ -29,4 +29,4 @@
     'menu': Menu,
 }
 
-LOADER = FileSystemLoader('./')
\ No newline at end of file
+LOADER = FileSystemLoader('./')
