Fix sonic-db-cli script to be compatible with python3 and python2.7 (s…

…onic-net#66)

* Fix sonic-db-cli script to be compatible with python3 and python2.7
* Review comments
* Removed space between function name and parentheses

src/swsssdk/scripts/sonic-db-cli

@@ -1,4 +1,5 @@
 #!/usr/bin/python
+from __future__ import print_function
 import sys
 import swsssdk
 import redis
@@ -28,10 +29,10 @@ def ping_all_instances():
         if rsp != 'PONG':
             msg.append(rsp)
     if msg:
-        print '\n'.join(msg)
+        print('\n'.join(msg))
         sys.exit(1)
     else:
-        print 'PONG'
+        print('PONG')
         sys.exit(0)
 
 def execute_cmd(dbname, cmd):
@@ -40,7 +41,7 @@ def execute_cmd(dbname, cmd):
         dbconn.connect(dbname)
     except RuntimeError:
         msg = "Invalid database name input : '{}'".format(dbname)
-        print >> sys.stderr, msg
+        print(msg, file=sys.stderr)
         sys.exit(1)
     else:
         client = dbconn.get_redis_client(dbname)
@@ -51,11 +52,11 @@ def execute_cmd(dbname, cmd):
         with these changes, it is enough for us to mimic redis-cli in SONiC so far since no application uses tty mode redis-cli output
         """
         if resp is None:
-            print ""
+            print()
         elif isinstance(resp, list):
-            print "\n".join(resp)
+            print("\n".join(resp))
         else:
-            print resp
+            print(resp)
         sys.exit(0)
 
 def main():