Fix bug: ConfigDBConnector.get_table does not work in python3 (sonic-…

…net#92)

`ConfigDBConnector` is not working well under python3, for example:
```
Oct 29 21:56:58.496505 sonic INFO hostcfgd[20662]: Traceback (most recent call last):
Oct 29 21:56:58.496724 sonic INFO hostcfgd[20662]:   File "/usr/bin/hostcfgd", line 387, in <module>
Oct 29 21:56:58.496873 sonic INFO hostcfgd[20662]:     main()
Oct 29 21:56:58.497014 sonic INFO hostcfgd[20662]:   File "/usr/bin/hostcfgd", line 382, in main
Oct 29 21:56:58.497147 sonic INFO hostcfgd[20662]:     daemon = HostConfigDaemon()
Oct 29 21:56:58.497939 sonic INFO hostcfgd[20662]:   File "/usr/bin/hostcfgd", line 237, in __init__
Oct 29 21:56:58.498103 sonic INFO hostcfgd[20662]:     tacacs_server = self.config_db.get_table('TACPLUS_SERVER')
Oct 29 21:56:58.498236 sonic INFO hostcfgd[20662]:   File "/usr/local/lib/python3.7/dist-packages/swsssdk/configdb.py", line 269, in get_table
Oct 29 21:56:58.498388 sonic INFO hostcfgd[20662]:     entry = self.raw_to_typed(client.hgetall(key))
Oct 29 21:56:58.498523 sonic INFO hostcfgd[20662]:   File "/usr/local/lib/python3.7/dist-packages/swsssdk/configdb.py", line 123, in raw_to_typed
Oct 29 21:56:58.498649 sonic INFO hostcfgd[20662]:     key = raw_key.decode()
Oct 29 21:56:58.498779 sonic INFO hostcfgd[20662]: AttributeError: 'str' object has no attribute 'decode'
```

So I will force parent class `SonicV2Connector` to handle all the `encode` / `decode` by specify `decode_responses=True` in its constructor parameters. But the class `ConfigDBConnector` will refuse contructed by `decode_responses=False`, which is okay to existing code base.

Tested in DUT:
```
Python 3.7.3 (default, Jul 25 2020, 13:03:44)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from swsssdk import ConfigDBConnector
>>> db = ConfigDBConnector()
>>> db.dbintf.redis_kwargs
{'host': '127.0.0.1', 'decode_responses': True}
>>> db.connect()
>>> db.get_table('TACPLUS_SERVER')
{'10.10.10.10': {'priority': '1', 'tcp_port': '49'}}
```

src/swsssdk/configdb.py

@@ -1,5 +1,5 @@
 """
-SONiC ConfigDB connection module 
+SONiC ConfigDB connection module
 
 Example:
     # Write to config DB
@@ -29,11 +29,16 @@ class ConfigDBConnector(SonicV2Connector):
     TABLE_NAME_SEPARATOR = '|'
     KEY_SEPARATOR = '|'
 
-    def __init__(self, **kwargs):
+    def __init__(self, decode_responses=True, **kwargs):
         # By default, connect to Redis through TCP, which does not requires root.
         if len(kwargs) == 0:
             kwargs['host'] = '127.0.0.1'
 
+        if PY3K:
+            if not decode_responses:
+                raise NotImplementedError('ConfigDBConnector with decode_responses=False is not supported in python3')
+            kwargs['decode_responses'] = True
+
         """The ConfigDBConnector class will accept the parameter 'namespace' which is used to
            load the database_config and connect to the redis DB instances in that namespace.
            By default namespace is set to None, which means it connects to local redis DB instances.
@@ -88,11 +93,11 @@ def unsubscribe(self, table):
         Args:
             table: Table name.
         """
-        if self.handlers.has_key(table):
+        if table in self.handlers:
             self.handlers.pop(table)
 
     def __fire(self, table, key, data):
-        if self.handlers.has_key(table):
+        if table in self.handlers:
             handler = self.handlers[table]
             handler(table, key, data)
 
@@ -106,7 +111,7 @@ def listen(self):
                 key = item['channel'].split(':', 1)[1]
                 try:
                     (table, row) = key.split(self.TABLE_NAME_SEPARATOR, 1)
-                    if self.handlers.has_key(table):
+                    if table in self.handlers:
                         client = self.get_redis_client(self.db_name)
                         data = self.raw_to_typed(client.hgetall(key))
                         self.__fire(table, row, data)
@@ -119,26 +124,17 @@ def raw_to_typed(self, raw_data):
         typed_data = {}
         for raw_key in raw_data:
             key = raw_key
-            if PY3K:
-                key = raw_key.decode()
 
             # "NULL:NULL" is used as a placeholder for objects with no attributes
             if key == "NULL":
                 pass
             # A column key with ending '@' is used to mark list-typed table items
             # TODO: Replace this with a schema-based typing mechanism.
             elif key.endswith("@"):
-                value = ""
-                if PY3K:
-                    value = raw_data[raw_key].decode("utf-8").split(',')
-                else:
-                    value = raw_data[raw_key].split(',')
+                value = raw_data[raw_key].split(',')
                 typed_data[key[:-1]] = value
             else:
-                if PY3K:
-                    typed_data[key] = raw_data[raw_key].decode()
-                else:
-                    typed_data[key] = raw_data[raw_key]
+                typed_data[key] = raw_data[raw_key]
         return typed_data
 
     def typed_to_raw(self, typed_data):
@@ -188,7 +184,7 @@ def set_entry(self, table, key, data):
         else:
             original = self.get_entry(table, key)
             client.hmset(_hash, self.typed_to_raw(data))
-            for k in [ k for k in original.keys() if k not in data.keys() ]:
+            for k in [ k for k in original if k not in data ]:
                 if type(original[k]) == list:
                     k = k + '@'
                 client.hdel(_hash, self.serialize_key(k))
@@ -239,8 +235,6 @@ def get_keys(self, table, split=True):
         data = []
         for key in keys:
             try:
-                if PY3K:
-                    key = key.decode()
                 if split:
                     (_, row) = key.split(self.TABLE_NAME_SEPARATOR, 1)
                     data.append(self.deserialize_key(row))
@@ -267,14 +261,9 @@ def get_table(self, table):
         for key in keys:
             try:
                 entry = self.raw_to_typed(client.hgetall(key))
-                if entry != None:
-                    if PY3K:
-                        key = key.decode()
-                        (_, row) = key.split(self.TABLE_NAME_SEPARATOR, 1)
-                        data[self.deserialize_key(row)] = entry
-                    else:
-                        (_, row) = key.split(self.TABLE_NAME_SEPARATOR, 1)
-                        data[self.deserialize_key(row)] = entry
+                if entry is not None:
+                    (_, row) = key.split(self.TABLE_NAME_SEPARATOR, 1)
+                    data[self.deserialize_key(row)] = entry
             except ValueError:
                 pass    #Ignore non table-formated redis entries
         return data
@@ -324,8 +313,6 @@ def get_config(self):
         keys = client.keys('*')
         data = {}
         for key in keys:
-            if PY3K:
-                key = key.decode()
             try:
                 (table_name, row) = key.split(self.TABLE_NAME_SEPARATOR, 1)
                 entry = self.raw_to_typed(client.hgetall(key))