diff --git a/ryu/lib/bfdlib.py b/ryu/lib/bfdlib.py
index 388618a7..6818b3e7 100644
--- a/ryu/lib/bfdlib.py
+++ b/ryu/lib/bfdlib.py
@@ -34,10 +34,11 @@ Please note that:
 
 
 import logging
-import six
 import time
 import random
 
+import six
+
 from ryu.base import app_manager
 from ryu.controller import event
 from ryu.controller import ofp_event
@@ -47,7 +48,6 @@ from ryu.exception import RyuException
 from ryu.ofproto.ether import ETH_TYPE_IP, ETH_TYPE_ARP
 from ryu.ofproto import ofproto_v1_3
 from ryu.ofproto import inet
-from ryu.lib import ofctl_v1_3
 from ryu.lib import hub
 from ryu.lib.packet import packet
 from ryu.lib.packet import ethernet
@@ -462,7 +462,6 @@ class BFDSession(object):
                 self._remote_session_state == bfd.BFD_STATE_UP:
             flags |= bfd.BFD_FLAG_DEMAND
 
-        ver = 1
         diag = self._local_diag
         state = self._session_state
         detect_mult = self._detect_mult
@@ -570,13 +569,13 @@ class BFDPacket(object):
         i = iter(pkt)
         eth_pkt = next(i)
 
-        assert type(eth_pkt) == ethernet.ethernet
+        assert isinstance(eth_pkt, ethernet.ethernet)
 
         ipv4_pkt = next(i)
-        assert type(ipv4_pkt) == ipv4.ipv4
+        assert isinstance(ipv4_pkt, ipv4.ipv4)
 
-        udp_pkt = i.next()
-        assert type(udp_pkt) == udp.udp
+        udp_pkt = next(i)
+        assert isinstance(udp_pkt, udp.udp)
 
         udp_payload = next(i)
 
@@ -619,10 +618,10 @@ class ARPPacket(object):
         i = iter(pkt)
         eth_pkt = next(i)
         # Ensure it's an ethernet frame.
-        assert type(eth_pkt) == ethernet.ethernet
+        assert isinstance(eth_pkt, ethernet.ethernet)
 
         arp_pkt = next(i)
-        if type(arp_pkt) != arp.arp:
+        if not isinstance(arp_pkt, arp.arp):
             raise ARPPacket.ARPUnknownFormat()
 
         if arp_pkt.opcode not in (ARP_REQUEST, ARP_REPLY):
diff --git a/ryu/lib/dpid.py b/ryu/lib/dpid.py
index 1224ae0e..87208859 100644
--- a/ryu/lib/dpid.py
+++ b/ryu/lib/dpid.py
@@ -18,8 +18,7 @@
 # string representation is in hex without '0x'
 
 _DPID_LEN = 16
-_DPID_LEN_STR = str(_DPID_LEN)
-_DPID_FMT = '%0' + _DPID_LEN_STR + 'x'
+_DPID_FMT = '%0{0}x'.format(_DPID_LEN)
 DPID_PATTERN = r'[0-9a-f]{%d}' % _DPID_LEN
 
 
diff --git a/ryu/lib/hub.py b/ryu/lib/hub.py
index 2ec8d691..a5ac50a6 100644
--- a/ryu/lib/hub.py
+++ b/ryu/lib/hub.py
@@ -54,9 +54,9 @@ if HUB_TYPE == 'eventlet':
                 return func(*args, **kwargs)
             except TaskExit:
                 pass
-            except:
+            except BaseException as e:
                 if raise_error:
-                    raise
+                    raise e
                 # Log uncaught exception.
                 # Note: this is an intentional divergence from gevent
                 # behaviour; gevent silently ignores such exceptions.
@@ -75,9 +75,9 @@ if HUB_TYPE == 'eventlet':
                 return func(*args, **kwargs)
             except TaskExit:
                 pass
-            except:
+            except BaseException as e:
                 if raise_error:
-                    raise
+                    raise e
                 # Log uncaught exception.
                 # Note: this is an intentional divergence from gevent
                 # behaviour; gevent silently ignores such exceptions.
diff --git a/ryu/lib/igmplib.py b/ryu/lib/igmplib.py
index a2822323..719626f3 100644
--- a/ryu/lib/igmplib.py
+++ b/ryu/lib/igmplib.py
@@ -573,7 +573,7 @@ class IgmpSnooper(IgmpBase):
             timeout = query.maxresp / 10
 
         self._to_hosts.setdefault(dpid, {})
-        if '0.0.0.0' == query.address:
+        if query.address == '0.0.0.0':
             # general query. reset all reply status.
             for group in self._to_hosts[dpid].values():
                 group['replied'] = False
diff --git a/ryu/lib/lacplib.py b/ryu/lib/lacplib.py
index 84d4777c..36682a6f 100644
--- a/ryu/lib/lacplib.py
+++ b/ryu/lib/lacplib.py
@@ -84,12 +84,11 @@ class LacpLib(app_manager.RyuApp):
         if you want to use multi LAG, call 'add' method more than once.
         """
         assert isinstance(ports, list)
-        assert 2 <= len(ports)
+        assert len(ports) >= 2
         ifs = {}
         for port in ports:
             ifs[port] = {'enabled': False, 'timeout': 0}
-        bond = {}
-        bond[dpid] = ifs
+        bond = {dpid: ifs}
         self._bonds.append(bond)
 
     # -------------------------------------------------------------------
diff --git a/ryu/lib/ofctl_utils.py b/ryu/lib/ofctl_utils.py
index 89cd5c83..a09517a2 100644
--- a/ryu/lib/ofctl_utils.py
+++ b/ryu/lib/ofctl_utils.py
@@ -195,7 +195,7 @@ def to_match_vid(value, ofpvid_present):
 def to_match_masked_int(value):
     if isinstance(value, str) and '/' in value:
         value = value.split('/')
-        return (str_to_int(value[0]), str_to_int(value[1]))
+        return str_to_int(value[0]), str_to_int(value[1])
 
     return str_to_int(value)
 
diff --git a/ryu/lib/ofctl_v1_0.py b/ryu/lib/ofctl_v1_0.py
index b38cc12b..be7d3913 100644
--- a/ryu/lib/ofctl_v1_0.py
+++ b/ryu/lib/ofctl_v1_0.py
@@ -290,8 +290,8 @@ def get_desc_stats(dp, waiters):
              'sw_desc': stats.sw_desc,
              'serial_num': stats.serial_num,
              'dp_desc': stats.dp_desc}
-    desc = {str(dp.id): s}
-    return desc
+
+    return {str(dp.id): s}
 
 
 def get_queue_stats(dp, waiters, port=None, queue_id=None):
@@ -319,8 +319,8 @@ def get_queue_stats(dp, waiters, port=None, queue_id=None):
                       'tx_bytes': stat.tx_bytes,
                       'tx_errors': stat.tx_errors,
                       'tx_packets': stat.tx_packets})
-    desc = {str(dp.id): s}
-    return desc
+
+    return {str(dp.id): s}
 
 
 def get_flow_stats(dp, waiters, flow=None):
@@ -355,8 +355,8 @@ def get_flow_stats(dp, waiters, flow=None):
                  'packet_count': stats.packet_count,
                  'table_id': UTIL.ofp_table_to_user(stats.table_id)}
             flows.append(s)
-    flows = {str(dp.id): flows}
-    return flows
+
+    return {str(dp.id): flows}
 
 
 def get_aggregate_flow_stats(dp, waiters, flow=None):
@@ -381,9 +381,8 @@ def get_aggregate_flow_stats(dp, waiters, flow=None):
                  'byte_count': st.byte_count,
                  'flow_count': st.flow_count}
             flows.append(s)
-    flows = {str(dp.id): flows}
 
-    return flows
+    return {str(dp.id): flows}
 
 
 def get_table_stats(dp, waiters):
@@ -432,9 +431,8 @@ def get_table_stats(dp, waiters):
                  'lookup_count': stat.lookup_count,
                  'matched_count': stat.matched_count}
             tables.append(s)
-    desc = {str(dp.id): tables}
 
-    return desc
+    return {str(dp.id): tables}
 
 
 def get_port_stats(dp, waiters, port=None):
@@ -465,8 +463,8 @@ def get_port_stats(dp, waiters, port=None):
                  'rx_crc_err': stats.rx_crc_err,
                  'collisions': stats.collisions}
             ports.append(s)
-    ports = {str(dp.id): ports}
-    return ports
+
+    return {str(dp.id): ports}
 
 
 def get_port_desc(dp, waiters):
@@ -476,7 +474,6 @@ def get_port_desc(dp, waiters):
     ofctl_utils.send_stats_request(dp, stats, waiters, msgs, LOG)
 
     descs = []
-
     for msg in msgs:
         stats = msg.ports
         for stat in stats.values():
@@ -490,8 +487,8 @@ def get_port_desc(dp, waiters):
                  'supported': stat.supported,
                  'peer': stat.peer}
             descs.append(d)
-    descs = {str(dp.id): descs}
-    return descs
+
+    return {str(dp.id): descs}
 
 
 def mod_flow_entry(dp, flow, cmd):
diff --git a/ryu/lib/ofctl_v1_2.py b/ryu/lib/ofctl_v1_2.py
index 3015b305..f51b0fd2 100644
--- a/ryu/lib/ofctl_v1_2.py
+++ b/ryu/lib/ofctl_v1_2.py
@@ -95,8 +95,8 @@ def to_actions(dp, acts):
             if action_type == 'WRITE_ACTIONS':
                 write_actions = []
                 write_acts = a.get('actions')
-                for a in write_acts:
-                    action = to_action(dp, a)
+                for act in write_acts:
+                    action = to_action(dp, act)
                     if action is not None:
                         write_actions.append(action)
                     else:
@@ -106,8 +106,8 @@ def to_actions(dp, acts):
                         parser.OFPInstructionActions(ofp.OFPIT_WRITE_ACTIONS,
                                                      write_actions))
             elif action_type == 'CLEAR_ACTIONS':
-                inst.append(parser.OFPInstructionActions(
-                            ofp.OFPIT_CLEAR_ACTIONS, []))
+                inst.append(
+                    parser.OFPInstructionActions(ofp.OFPIT_CLEAR_ACTIONS, []))
             elif action_type == 'GOTO_TABLE':
                 table_id = UTIL.ofp_table_from_user(a.get('table_id'))
                 inst.append(parser.OFPInstructionGotoTable(table_id))
@@ -358,8 +358,7 @@ def match_to_str(ofmatch):
             'tcp_src': 'tp_src',
             'tcp_dst': 'tp_dst',
             'udp_src': 'tp_src',
-            'udp_dst': 'tp_dst'
-            }
+            'udp_dst': 'tp_dst'}
 
     match = {}
 
@@ -407,8 +406,8 @@ def get_desc_stats(dp, waiters):
              'sw_desc': stats.sw_desc,
              'serial_num': stats.serial_num,
              'dp_desc': stats.dp_desc}
-    desc = {str(dp.id): s}
-    return desc
+
+    return {str(dp.id): s}
 
 
 def get_queue_stats(dp, waiters, port=None, queue_id=None):
@@ -438,8 +437,8 @@ def get_queue_stats(dp, waiters, port=None, queue_id=None):
                       'tx_bytes': stat.tx_bytes,
                       'tx_errors': stat.tx_errors,
                       'tx_packets': stat.tx_packets})
-    desc = {str(dp.id): s}
-    return desc
+
+    return {str(dp.id): s}
 
 
 def get_queue_config(dp, waiters, port=None):
@@ -452,10 +451,11 @@ def get_queue_config(dp, waiters, port=None):
     msgs = []
     ofctl_utils.send_stats_request(dp, stats, waiters, msgs, LOG)
 
-    prop_type = {dp.ofproto.OFPQT_MIN_RATE: 'MIN_RATE',
-                 dp.ofproto.OFPQT_MAX_RATE: 'MAX_RATE',
-                 dp.ofproto.OFPQT_EXPERIMENTER: 'EXPERIMENTER',
-                 }
+    prop_type = {
+        dp.ofproto.OFPQT_MIN_RATE: 'MIN_RATE',
+        dp.ofproto.OFPQT_MAX_RATE: 'MAX_RATE',
+        dp.ofproto.OFPQT_EXPERIMENTER: 'EXPERIMENTER',
+    }
 
     configs = []
     for config in msgs:
@@ -478,9 +478,8 @@ def get_queue_config(dp, waiters, port=None):
         c = {'port': UTIL.ofp_port_to_user(config.port),
              'queues': queue_list}
         configs.append(c)
-    configs = {str(dp.id): configs}
 
-    return configs
+    return {str(dp.id): configs}
 
 
 def get_flow_stats(dp, waiters, flow=None):
@@ -519,9 +518,8 @@ def get_flow_stats(dp, waiters, flow=None):
                  'table_id': UTIL.ofp_table_to_user(stats.table_id),
                  'length': stats.length}
             flows.append(s)
-    flows = {str(dp.id): flows}
 
-    return flows
+    return {str(dp.id): flows}
 
 
 def get_aggregate_flow_stats(dp, waiters, flow=None):
@@ -549,9 +547,8 @@ def get_aggregate_flow_stats(dp, waiters, flow=None):
              'byte_count': stats.byte_count,
              'flow_count': stats.flow_count}
         flows.append(s)
-    flows = {str(dp.id): flows}
 
-    return flows
+    return {str(dp.id): flows}
 
 
 def get_table_stats(dp, waiters):
@@ -674,9 +671,8 @@ def get_table_stats(dp, waiters):
                  'lookup_count': stat.lookup_count,
                  'matched_count': stat.matched_count}
             tables.append(s)
-    desc = {str(dp.id): tables}
 
-    return desc
+    return {str(dp.id): tables}
 
 
 def get_port_stats(dp, waiters, port=None):
@@ -707,8 +703,8 @@ def get_port_stats(dp, waiters, port=None):
                  'rx_crc_err': stats.rx_crc_err,
                  'collisions': stats.collisions}
             ports.append(s)
-    ports = {str(dp.id): ports}
-    return ports
+
+    return {str(dp.id): ports}
 
 
 def get_group_stats(dp, waiters, group_id=None):
@@ -737,8 +733,8 @@ def get_group_stats(dp, waiters, group_id=None):
                  'byte_count': stats.byte_count,
                  'bucket_stats': bucket_counters}
             groups.append(g)
-    groups = {str(dp.id): groups}
-    return groups
+
+    return {str(dp.id): groups}
 
 
 def get_group_features(dp, waiters):
@@ -797,8 +793,8 @@ def get_group_features(dp, waiters):
              'max_groups': max_groups,
              'actions': actions}
         features.append(f)
-    features = {str(dp.id): features}
-    return features
+
+    return {str(dp.id): features}
 
 
 def get_group_desc(dp, waiters):
@@ -829,8 +825,8 @@ def get_group_desc(dp, waiters):
                  'group_id': UTIL.ofp_group_to_user(stats.group_id),
                  'buckets': buckets}
             descs.append(d)
-    descs = {str(dp.id): descs}
-    return descs
+
+    return {str(dp.id): descs}
 
 
 def get_port_desc(dp, waiters):
@@ -856,8 +852,8 @@ def get_port_desc(dp, waiters):
                  'curr_speed': stat.curr_speed,
                  'max_speed': stat.max_speed}
             descs.append(d)
-    descs = {str(dp.id): descs}
-    return descs
+
+    return {str(dp.id): descs}
 
 
 def mod_flow_entry(dp, flow, cmd):
diff --git a/ryu/lib/ofctl_v1_3.py b/ryu/lib/ofctl_v1_3.py
index 3d768ce2..f407e5ad 100644
--- a/ryu/lib/ofctl_v1_3.py
+++ b/ryu/lib/ofctl_v1_3.py
@@ -55,8 +55,8 @@ def to_actions(dp, acts):
             if action_type == 'WRITE_ACTIONS':
                 write_actions = []
                 write_acts = a.get('actions')
-                for a in write_acts:
-                    action = to_action(dp, a)
+                for act in write_acts:
+                    action = to_action(dp, act)
                     if action is not None:
                         write_actions.append(action)
                     else:
@@ -66,8 +66,8 @@ def to_actions(dp, acts):
                         parser.OFPInstructionActions(ofp.OFPIT_WRITE_ACTIONS,
                                                      write_actions))
             elif action_type == 'CLEAR_ACTIONS':
-                inst.append(parser.OFPInstructionActions(
-                            ofp.OFPIT_CLEAR_ACTIONS, []))
+                inst.append(
+                    parser.OFPInstructionActions(ofp.OFPIT_CLEAR_ACTIONS, []))
             elif action_type == 'GOTO_TABLE':
                 table_id = UTIL.ofp_table_from_user(a.get('table_id'))
                 inst.append(parser.OFPInstructionGotoTable(table_id))
@@ -133,7 +133,7 @@ def action_to_str(act):
         if act.experimenter == ofproto_common.NX_EXPERIMENTER_ID:
             try:
                 return ofctl_nicira_ext.action_to_str(act, action_to_str)
-            except:
+            except Exception:
                 LOG.debug('Error parsing NX_ACTION(%s)',
                           act.__class__.__name__, exc_info=True)
 
@@ -299,8 +299,7 @@ def match_to_str(ofmatch):
             'tcp_src': 'tp_src',
             'tcp_dst': 'tp_dst',
             'udp_src': 'tp_src',
-            'udp_dst': 'tp_dst'
-            }
+            'udp_dst': 'tp_dst'}
 
     match = {}
 
@@ -395,8 +394,7 @@ def get_queue_config(dp, waiters, port=None, to_user=True):
 
     prop_type = {dp.ofproto.OFPQT_MIN_RATE: 'MIN_RATE',
                  dp.ofproto.OFPQT_MAX_RATE: 'MAX_RATE',
-                 dp.ofproto.OFPQT_EXPERIMENTER: 'EXPERIMENTER',
-                 }
+                 dp.ofproto.OFPQT_EXPERIMENTER: 'EXPERIMENTER'}
 
     configs = []
     for config in msgs:
@@ -564,8 +562,7 @@ def get_table_features(dp, waiters, to_user=True):
                  ofproto.OFPTFPT_APPLY_SETFIELD: 'APPLY_SETFIELD',
                  ofproto.OFPTFPT_APPLY_SETFIELD_MISS: 'APPLY_SETFIELD_MISS',
                  ofproto.OFPTFPT_EXPERIMENTER: 'EXPERIMENTER',
-                 ofproto.OFPTFPT_EXPERIMENTER_MISS: 'EXPERIMENTER_MISS'
-                 }
+                 ofproto.OFPTFPT_EXPERIMENTER_MISS: 'EXPERIMENTER_MISS'}
 
     if not to_user:
         prop_type = dict((k, k) for k in prop_type.keys())
@@ -600,41 +597,42 @@ def get_table_features(dp, waiters, to_user=True):
                 p = {'type': prop_type.get(prop.type, 'UNKNOWN')}
                 if prop.type in p_type_instructions:
                     instruction_ids = []
-                    for id in prop.instruction_ids:
-                        i = {'len': id.len,
-                             'type': id.type}
-                        instruction_ids.append(i)
+                    for i in prop.instruction_ids:
+                        inst = {'len': i.len,
+                                'type': i.type}
+                        instruction_ids.append(inst)
                     p['instruction_ids'] = instruction_ids
                 elif prop.type in p_type_next_tables:
                     table_ids = []
-                    for id in prop.table_ids:
-                        table_ids.append(id)
+                    for i in prop.table_ids:
+                        table_ids.append(i)
                     p['table_ids'] = table_ids
                 elif prop.type in p_type_actions:
                     action_ids = []
-                    for id in prop.action_ids:
-                        i = {'len': id.len,
-                             'type': id.type}
-                        action_ids.append(i)
+                    for i in prop.action_ids:
+                        act = {'len': i.len,
+                               'type': i.type}
+                        action_ids.append(act)
                     p['action_ids'] = action_ids
                 elif prop.type in p_type_oxms:
                     oxm_ids = []
-                    for id in prop.oxm_ids:
-                        i = {'hasmask': id.hasmask,
-                             'length': id.length,
-                             'type': id.type}
-                        oxm_ids.append(i)
+                    for i in prop.oxm_ids:
+                        oxm = {'hasmask': i.hasmask,
+                               'length': i.length,
+                               'type': i.type}
+                        oxm_ids.append(oxm)
                     p['oxm_ids'] = oxm_ids
                 elif prop.type in p_type_experimenter:
                     pass
                 properties.append(p)
-            s = {'name': stat.name.decode('utf-8'),
-                 'metadata_match': stat.metadata_match,
-                 'metadata_write': stat.metadata_write,
-                 'config': stat.config,
-                 'max_entries': stat.max_entries,
-                 'properties': properties,
-                 }
+            s = {
+                'name': stat.name.decode('utf-8'),
+                'metadata_match': stat.metadata_match,
+                'metadata_write': stat.metadata_write,
+                'config': stat.config,
+                'max_entries': stat.max_entries,
+                'properties': properties,
+            }
 
             if to_user:
                 s['table_id'] = UTIL.ofp_table_to_user(stat.table_id)
diff --git a/ryu/lib/ofctl_v1_4.py b/ryu/lib/ofctl_v1_4.py
index 98ac94d8..71e33fd7 100644
--- a/ryu/lib/ofctl_v1_4.py
+++ b/ryu/lib/ofctl_v1_4.py
@@ -166,8 +166,7 @@ def to_match(dp, attrs):
                'pbb_isid': ofctl_utils.to_match_masked_int,
                'tunnel_id': ofctl_utils.to_match_masked_int,
                'ipv6_exthdr': ofctl_utils.to_match_masked_int,
-               'pbb_uca': int,
-               }
+               'pbb_uca': int}
 
     keys = {'dl_dst': 'eth_dst',
             'dl_src': 'eth_src',
@@ -430,27 +429,27 @@ def get_table_features(dp, waiters, to_user=True):
                 p['type'] = t if t != prop.type else 'UNKNOWN'
                 if prop.type in p_type_instructions:
                     instruction_ids = []
-                    for id in prop.instruction_ids:
-                        i = {'len': id.len,
-                             'type': id.type}
-                        instruction_ids.append(i)
+                    for i in prop.instruction_ids:
+                        inst = {'len': i.len,
+                                'type': i.type}
+                        instruction_ids.append(inst)
                     p['instruction_ids'] = instruction_ids
                 elif prop.type in p_type_next_tables:
                     table_ids = []
-                    for id in prop.table_ids:
-                        table_ids.append(id)
+                    for i in prop.table_ids:
+                        table_ids.append(i)
                     p['table_ids'] = table_ids
                 elif prop.type in p_type_actions:
                     action_ids = []
-                    for id in prop.action_ids:
-                        i = id.to_jsondict()[id.__class__.__name__]
-                        action_ids.append(i)
+                    for i in prop.action_ids:
+                        act = i.to_jsondict()[i.__class__.__name__]
+                        action_ids.append(act)
                     p['action_ids'] = action_ids
                 elif prop.type in p_type_oxms:
                     oxm_ids = []
-                    for id in prop.oxm_ids:
-                        i = id.to_jsondict()[id.__class__.__name__]
-                        oxm_ids.append(i)
+                    for i in prop.oxm_ids:
+                        oxm = i.to_jsondict()[i.__class__.__name__]
+                        oxm_ids.append(oxm)
                     p['oxm_ids'] = oxm_ids
                 elif prop.type in p_type_experimenter:
                     pass
@@ -677,8 +676,7 @@ def get_group_features(dp, waiters, to_user=True):
                    ofp.OFPAT_SET_FIELD: 'SET_FIELD',
                    ofp.OFPAT_PUSH_PBB: 'PUSH_PBB',
                    ofp.OFPAT_POP_PBB: 'POP_PBB',
-                   ofp.OFPAT_EXPERIMENTER: 'EXPERIMENTER',
-                   }
+                   ofp.OFPAT_EXPERIMENTER: 'EXPERIMENTER'}
 
     stats = dp.ofproto_parser.OFPGroupFeaturesStatsRequest(dp, 0)
     msgs = []
diff --git a/ryu/lib/ofctl_v1_5.py b/ryu/lib/ofctl_v1_5.py
index 52c2de88..43ee0fcb 100644
--- a/ryu/lib/ofctl_v1_5.py
+++ b/ryu/lib/ofctl_v1_5.py
@@ -171,8 +171,7 @@ def to_match(dp, attrs):
                'pbb_uca': int,
                'tcp_flags': int,
                'actset_output': int,
-               'packet_type': ofctl_utils.to_match_packet_type,
-               }
+               'packet_type': ofctl_utils.to_match_packet_type}
 
     keys = {'dl_dst': 'eth_dst',
             'dl_src': 'eth_src',
@@ -501,27 +500,27 @@ def get_table_features(dp, waiters, to_user=True):
                 p['type'] = t if t != prop.type else 'UNKNOWN'
                 if prop.type in p_type_instructions:
                     instruction_ids = []
-                    for id in prop.instruction_ids:
-                        i = {'len': id.len,
-                             'type': id.type}
-                        instruction_ids.append(i)
+                    for i in prop.instruction_ids:
+                        inst = {'len': i.len,
+                                'type': i.type}
+                        instruction_ids.append(inst)
                     p['instruction_ids'] = instruction_ids
                 elif prop.type in p_type_next_tables:
                     table_ids = []
-                    for id in prop.table_ids:
-                        table_ids.append(id)
+                    for i in prop.table_ids:
+                        table_ids.append(i)
                     p['table_ids'] = table_ids
                 elif prop.type in p_type_actions:
                     action_ids = []
-                    for id in prop.action_ids:
-                        i = id.to_jsondict()[id.__class__.__name__]
-                        action_ids.append(i)
+                    for i in prop.action_ids:
+                        act = i.to_jsondict()[i.__class__.__name__]
+                        action_ids.append(act)
                     p['action_ids'] = action_ids
                 elif prop.type in p_type_oxms:
                     oxm_ids = []
-                    for id in prop.oxm_ids:
-                        i = id.to_jsondict()[id.__class__.__name__]
-                        oxm_ids.append(i)
+                    for i in prop.oxm_ids:
+                        oxm = i.to_jsondict()[i.__class__.__name__]
+                        oxm_ids.append(oxm)
                     p['oxm_ids'] = oxm_ids
                 elif prop.type == p_type_packet:
                     oxm_values = []
@@ -756,8 +755,7 @@ def get_group_features(dp, waiters, to_user=True):
                    ofp.OFPAT_POP_PBB: 'POP_PBB',
                    ofp.OFPAT_COPY_FIELD: 'COPY_FIELD',
                    ofp.OFPAT_METER: 'METER',
-                   ofp.OFPAT_EXPERIMENTER: 'EXPERIMENTER',
-                   }
+                   ofp.OFPAT_EXPERIMENTER: 'EXPERIMENTER'}
 
     stats = dp.ofproto_parser.OFPGroupFeaturesStatsRequest(dp, 0)
     msgs = []
diff --git a/ryu/lib/pack_utils.py b/ryu/lib/pack_utils.py
index a84d14a7..e2afa7b4 100644
--- a/ryu/lib/pack_utils.py
+++ b/ryu/lib/pack_utils.py
@@ -18,13 +18,6 @@ import struct
 
 
 def msg_pack_into(fmt, buf, offset, *args):
-    if len(buf) < offset:
-        buf += bytearray(offset - len(buf))
-
-    if len(buf) == offset:
-        buf += struct.pack(fmt, *args)
-        return
-
     needed_len = offset + struct.calcsize(fmt)
     if len(buf) < needed_len:
         buf += bytearray(needed_len - len(buf))
diff --git a/ryu/lib/port_no.py b/ryu/lib/port_no.py
index 33720f3c..0eb65635 100644
--- a/ryu/lib/port_no.py
+++ b/ryu/lib/port_no.py
@@ -17,8 +17,7 @@
 # string representation is in hex without '0x'
 
 _PORT_NO_LEN = 8
-_PORT_NO_LEN_STR = str(_PORT_NO_LEN)
-_PORT_NO_FMT = '%0' + _PORT_NO_LEN_STR + 'x'
+_PORT_NO_FMT = '%0{0}x'.format(_PORT_NO_LEN)
 PORT_NO_PATTERN = r'[0-9a-f]{%d}' % _PORT_NO_LEN
 
 
diff --git a/ryu/lib/rpc.py b/ryu/lib/rpc.py
index 57ba00c9..ed38f976 100644
--- a/ryu/lib/rpc.py
+++ b/ryu/lib/rpc.py
@@ -1,5 +1,3 @@
-#!/usr/bin/env python
-#
 # Copyright (C) 2013 Nippon Telegraph and Telephone Corporation.
 # Copyright (C) 2013 YAMAMOTO Takashi <yamamoto at valinux co jp>
 #
@@ -19,6 +17,9 @@
 # msgpack-rpc
 # http://wiki.msgpack.org/display/MSGPACK/RPC+specification
 
+from collections import deque
+import select
+
 import msgpack
 import six
 
@@ -53,12 +54,12 @@ class MessageEncoder(object):
         assert isinstance(method, six.binary_type)
         assert isinstance(params, list)
         msgid = self._create_msgid()
-        return (self._packer.pack([MessageType.REQUEST, msgid, method,
-                                  params]), msgid)
+        return (self._packer.pack(
+            [MessageType.REQUEST, msgid, method, params]), msgid)
 
     def create_response(self, msgid, error=None, result=None):
         assert isinstance(msgid, int)
-        assert 0 <= msgid and msgid <= 0xffffffff
+        assert 0 <= msgid <= 0xffffffff
         assert error is None or result is None
         return self._packer.pack([MessageType.RESPONSE, msgid, error, result])
 
@@ -76,21 +77,18 @@ class MessageEncoder(object):
         for m in self._unpacker:
             self._dispatch_message(m, disp_table)
 
-    def _dispatch_message(self, m, disp_table):
+    @staticmethod
+    def _dispatch_message(m, disp_table):
         # XXX validation
-        type = m[0]
+        t = m[0]
         try:
-            f = disp_table[type]
+            f = disp_table[t]
         except KeyError:
             # ignore messages with unknown type
             return
         f(m[1:])
 
 
-from collections import deque
-import select
-
-
 class EndPoint(object):
     """An endpoint
     *sock* is a socket-like.  it can be either blocking or non-blocking.
@@ -231,7 +229,7 @@ class EndPoint(object):
         except KeyError:
             return None
         error, result = m
-        return (result, error)
+        return result, error
 
     def get_notification(self):
         return self._get_message(self._notifications)
@@ -241,6 +239,7 @@ class RPCError(Exception):
     """an error from server
     """
     def __init__(self, error):
+        super(RPCError, self).__init__()
         self._error = error
 
     def get_value(self):
diff --git a/ryu/lib/snortlib.py b/ryu/lib/snortlib.py
index 803eb452..a8c1363e 100644
--- a/ryu/lib/snortlib.py
+++ b/ryu/lib/snortlib.py
@@ -40,6 +40,8 @@ class SnortLib(app_manager.RyuApp):
         self.name = 'snortlib'
         self.config = {'unixsock': True}
         self._set_logger()
+        self.sock = None
+        self.nwsock = None
 
     def set_config(self, config):
         assert isinstance(config, dict)
diff --git a/ryu/lib/sockopt.py b/ryu/lib/sockopt.py
index 6e533586..c3e9d1ad 100644
--- a/ryu/lib/sockopt.py
+++ b/ryu/lib/sockopt.py
@@ -21,6 +21,10 @@ import struct
 from ryu.lib import sockaddr
 
 
+TCP_MD5SIG_LINUX = 0x0e
+TCP_MD5SIG_BSD = 0x10
+
+
 def _set_tcp_md5sig_linux(s, addr, key):
     # struct tcp_md5sig {
     #     struct sockaddr_storage addr;
@@ -29,7 +33,6 @@ def _set_tcp_md5sig_linux(s, addr, key):
     #     u32 pad2;
     #     u8 key[80];
     # }
-    TCP_MD5SIG = 14
     af = s.family
     if af == socket.AF_INET:
         sa = sockaddr.sa_in4(addr)
@@ -39,14 +42,13 @@ def _set_tcp_md5sig_linux(s, addr, key):
         raise ValueError("unsupported af %s" % (af,))
     ss = sockaddr.sa_to_ss(sa)
     tcp_md5sig = ss + struct.pack("2xH4x80s", len(key), key)
-    s.setsockopt(socket.IPPROTO_TCP, TCP_MD5SIG, tcp_md5sig)
+    s.setsockopt(socket.IPPROTO_TCP, TCP_MD5SIG_LINUX, tcp_md5sig)
 
 
 def _set_tcp_md5sig_bsd(s, _addr, _key):
     # NOTE: On this platform, address and key need to be set using setkey(8).
-    TCP_MD5SIG = 0x10
     tcp_md5sig = struct.pack("I", 1)
-    s.setsockopt(socket.IPPROTO_TCP, TCP_MD5SIG, tcp_md5sig)
+    s.setsockopt(socket.IPPROTO_TCP, TCP_MD5SIG_BSD, tcp_md5sig)
 
 
 def set_tcp_md5sig(s, addr, key):
diff --git a/ryu/lib/stplib.py b/ryu/lib/stplib.py
index beb68e14..bb5bd6f0 100644
--- a/ryu/lib/stplib.py
+++ b/ryu/lib/stplib.py
@@ -191,7 +191,7 @@ class Stp(app_manager.RyuApp):
         self.bridge_list = {}
 
     def close(self):
-        for dpid in self.bridge_list.keys():
+        for dpid in self.bridge_list:
             self._unregister_bridge(dpid)
 
     def _set_logger(self):
@@ -480,12 +480,12 @@ class Bridge(object):
 
         pkt = packet.Packet(msg.data)
         if bpdu.ConfigurationBPDUs in pkt:
-            """ Receive Configuration BPDU.
-                 - If receive superior BPDU:
-                    re-caluculation of spanning tree.
-                 - If receive Topology Change BPDU:
-                    throw EventTopologyChange.
-                    forward Topology Change BPDU. """
+            # Received Configuration BPDU.
+            # - If received superior BPDU:
+            #    Re-calculates spanning tree.
+            # - If received Topology Change BPDU:
+            #    Throws EventTopologyChange.
+            #    Forwards Topology Change BPDU.
             (bpdu_pkt, ) = pkt.get_protocols(bpdu.ConfigurationBPDUs)
             if bpdu_pkt.message_age > bpdu_pkt.max_age:
                 log_msg = 'Drop BPDU packet which message_age exceeded.'
@@ -506,24 +506,23 @@ class Bridge(object):
                 self._forward_tc_bpdu(rcv_tc)
 
         elif bpdu.TopologyChangeNotificationBPDUs in pkt:
-            """ Receive Topology Change Notification BPDU.
-                 send Topology Change Ack BPDU.
-                 throw EventTopologyChange.
-                 - Root bridge:
-                    send Topology Change BPDU from all port.
-                 - Non root bridge:
-                    send Topology Change Notification BPDU to root bridge. """
+            # Received Topology Change Notification BPDU.
+            # Send Topology Change Ack BPDU and throws EventTopologyChange.
+            # - Root bridge:
+            #    Sends Topology Change BPDU from all port.
+            # - Non root bridge:
+            #    Sends Topology Change Notification BPDU to root bridge.
             in_port.transmit_ack_bpdu()
             self.topology_change_notify(None)
 
         elif bpdu.RstBPDUs in pkt:
-            """ Receive Rst BPDU. """
+            # Received Rst BPDU.
             # TODO: RSTP
             pass
 
         else:
-            """ Receive non BPDU packet.
-                 throw EventPacketIn. """
+            # Received non BPDU packet.
+            # Throws EventPacketIn.
             self.send_event(EventPacketIn(msg))
 
     def recalculate_spanning_tree(self, init=True):
@@ -544,7 +543,7 @@ class Bridge(object):
 
         if init:
             self.logger.info('Root bridge.', extra=self.dpid_str)
-            for port_no in self.ports.keys():
+            for port_no in self.ports:
                 port_roles[port_no] = DESIGNATED_PORT
         else:
             (port_roles,
@@ -574,7 +573,7 @@ class Bridge(object):
             root_priority = self.root_priority
             root_times = self.root_times
 
-            for port_no in self.ports.keys():
+            for port_no in self.ports:
                 if self.ports[port_no].state is not PORT_STATE_DISABLE:
                     port_roles[port_no] = DESIGNATED_PORT
         else:
@@ -726,7 +725,7 @@ class Port(object):
                       dp.ofproto.OFPPF_1GB_HD: bpdu.PORT_PATH_COST_1GB,
                       dp.ofproto.OFPPF_1GB_FD: bpdu.PORT_PATH_COST_1GB,
                       dp.ofproto.OFPPF_10GB_FD: bpdu.PORT_PATH_COST_10GB}
-        for rate in sorted(path_costs.keys(), reverse=True):
+        for rate in sorted(path_costs, reverse=True):
             if ofport.curr & rate:
                 values['path_cost'] = path_costs[rate]
                 break
@@ -859,10 +858,10 @@ class Port(object):
         if new_state is not PORT_STATE_DISABLE:
             self.ofctl.set_port_status(self.ofport, new_state)
 
-        if(new_state is PORT_STATE_FORWARD or
-                (self.state is PORT_STATE_FORWARD and
-                    (new_state is PORT_STATE_DISABLE or
-                     new_state is PORT_STATE_BLOCK))):
+        if(new_state is PORT_STATE_FORWARD
+           or (self.state is PORT_STATE_FORWARD
+               and (new_state is PORT_STATE_DISABLE
+                    or new_state is PORT_STATE_BLOCK))):
             self.topology_change_notify(new_state)
 
         if (new_state is PORT_STATE_DISABLE
@@ -927,7 +926,7 @@ class Port(object):
                      or self.role is NON_DESIGNATED_PORT)):
             self._update_wait_bpdu_timer()
             chk_flg = True
-        elif(rcv_info is INFERIOR and self.role is DESIGNATED_PORT):
+        elif rcv_info is INFERIOR and self.role is DESIGNATED_PORT:
             chk_flg = True
 
         # Check TopologyChange flag.
diff --git a/ryu/lib/stringify.py b/ryu/lib/stringify.py
index 21b0d9de..81eb6178 100644
--- a/ryu/lib/stringify.py
+++ b/ryu/lib/stringify.py
@@ -1,5 +1,3 @@
-#!/usr/bin/env python
-#
 # Copyright (C) 2013 Nippon Telegraph and Telephone Corporation.
 # Copyright (C) 2013 YAMAMOTO Takashi <yamamoto at valinux co jp>
 #
@@ -19,7 +17,6 @@
 from __future__ import print_function
 
 import base64
-import collections
 import inspect
 
 import six
@@ -104,7 +101,7 @@ class NXFlowSpecFieldType(TypeDescr):
         if not isinstance(v, list):
             return v
         field, ofs = v
-        return (field, ofs)
+        return field, ofs
 
 
 _types = {
@@ -126,7 +123,7 @@ class StringifyMixin(object):
     Currently the following types are implemented.
 
     ========= =============
-    Type      Descrption
+    Type      Description
     ========= =============
     ascii     US-ASCII
     utf-8     UTF-8
@@ -162,7 +159,7 @@ class StringifyMixin(object):
     def _is_class(cls, dict_):
         # we distinguish a dict like OFPSwitchFeatures.ports
         # from OFPxxx classes using heuristics.
-        # exmples of OFP classes:
+        # Examples of OFP classes:
         #   {"OFPMatch": { ... }}
         #   {"MTIPv6SRC": { ... }}
         assert isinstance(dict_, dict)
@@ -208,7 +205,7 @@ class StringifyMixin(object):
                 if six.PY3:
                     json_value = json_value.decode('ascii')
             elif isinstance(v, list):
-                json_value = list(map(_encode, v))
+                json_value = [_encode(ve) for ve in v]
             elif isinstance(v, dict):
                 json_value = _mapdict(_encode, v)
                 # while a python dict key can be any hashable object,
@@ -218,7 +215,7 @@ class StringifyMixin(object):
             else:
                 try:
                     json_value = v.to_jsondict()
-                except:
+                except Exception:
                     json_value = v
             return json_value
         return _encode
@@ -253,7 +250,7 @@ class StringifyMixin(object):
         =============  =====================================================
         """
         dict_ = {}
-        encode = lambda k, x: self._encode_value(k, x, encode_string)
+        encode = lambda key, val: self._encode_value(key, val, encode_string)
         for k, v in obj_attrs(self):
             dict_[k] = encode(k, v)
         return {self.__class__.__name__: dict_}
@@ -282,6 +279,8 @@ class StringifyMixin(object):
     @classmethod
     def _decode_value(cls, k, json_value, decode_string=base64.b64decode,
                       **additional_args):
+        # Note: To avoid passing redundant arguments (e.g. 'datapath' for
+        # non OFP classes), we omit '**additional_args' here.
         return cls._get_decoder(k, decode_string)(json_value)
 
     @classmethod
@@ -290,13 +289,13 @@ class StringifyMixin(object):
             if isinstance(json_value, (bytes, six.text_type)):
                 v = decode_string(json_value)
             elif isinstance(json_value, list):
-                v = list(map(_decode, json_value))
+                v = [_decode(jv) for jv in json_value]
             elif isinstance(json_value, dict):
                 if cls._is_class(json_value):
                     v = cls.obj_from_jsondict(json_value, **additional_args)
                 else:
                     v = _mapdict(_decode, json_value)
-                    # XXXhack
+                    # XXX: Hack
                     # try to restore integer keys used by
                     # OFPSwitchFeatures.ports.
                     try:
@@ -386,11 +385,11 @@ def obj_attrs(msg_):
     """
 
     if isinstance(msg_, StringifyMixin):
-        iter = msg_.stringify_attrs()
+        itr = msg_.stringify_attrs()
     else:
         # probably called by msg_str_attr
-        iter = obj_python_attrs(msg_)
-    for k, v in iter:
+        itr = obj_python_attrs(msg_)
+    for k, v in itr:
         if k.endswith('_') and k[:-1] in _RESERVED_KEYWORD:
             # XXX currently only StringifyMixin has restoring logic
             assert isinstance(msg_, StringifyMixin)
diff --git a/ryu/lib/type_desc.py b/ryu/lib/type_desc.py
index 3f158fc2..eca80137 100644
--- a/ryu/lib/type_desc.py
+++ b/ryu/lib/type_desc.py
@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import base64
 
 import six
 
@@ -28,20 +29,20 @@ class IntDescr(TypeDescr):
     def __init__(self, size):
         self.size = size
 
-    def to_user(self, bin):
+    def to_user(self, binary):
         i = 0
-        for x in range(self.size):
-            c = bin[:1]
+        for _ in range(self.size):
+            c = binary[:1]
             i = i * 256 + ord(c)
-            bin = bin[1:]
+            binary = binary[1:]
         return i
 
     def from_user(self, i):
-        bin = b''
-        for x in range(self.size):
-            bin = six.int2byte(i & 255) + bin
+        binary = b''
+        for _ in range(self.size):
+            binary = six.int2byte(i & 255) + binary
             i //= 256
-        return bin
+        return binary
 
 Int1 = IntDescr(1)
 Int2 = IntDescr(2)
@@ -65,13 +66,13 @@ class IntDescrMlt(TypeDescr):
         self.num = num
         self.size = length * num
 
-    def to_user(self, bin):
-        assert len(bin) == self.size
-        lb = _split_str(bin, self.length)
+    def to_user(self, binary):
+        assert len(binary) == self.size
+        lb = _split_str(binary, self.length)
         li = []
         for b in lb:
             i = 0
-            for x in range(self.length):
+            for _ in range(self.length):
                 c = b[:1]
                 i = i * 256 + ord(c)
                 b = b[1:]
@@ -80,14 +81,14 @@ class IntDescrMlt(TypeDescr):
 
     def from_user(self, li):
         assert len(li) == self.num
-        bin = b''
+        binary = b''
         for i in li:
             b = b''
-            for x in range(self.length):
+            for _ in range(self.length):
                 b = six.int2byte(i & 255) + b
                 i //= 256
-            bin += b
-        return bin
+            binary += b
+        return binary
 
 Int4Double = IntDescrMlt(4, 2)
 
@@ -111,13 +112,12 @@ class IPv6Addr(TypeDescr):
 
 
 class UnknownType(TypeDescr):
-    import base64
 
-    b64encode = base64.b64encode
-    if six.PY3:
-        @classmethod
-        def to_user(cls, data):
-            return cls.b64encode(data).decode('ascii')
-    else:
-        to_user = staticmethod(base64.b64encode)
+    @staticmethod
+    def to_user(data):
+        if six.PY3:
+            return base64.b64encode(data).decode('ascii')
+        else:
+            return base64.b64encode(data)
+
     from_user = staticmethod(base64.b64decode)