diff --git a/test/functional/interface_bitcoin_cli.py b/test/functional/interface_bitcoin_cli.py
--- a/test/functional/interface_bitcoin_cli.py
+++ b/test/functional/interface_bitcoin_cli.py
@@ -32,15 +32,15 @@
 
         self.log.info("Test -stdinrpcpass option")
         assert_equal(0, self.nodes[0].cli(
-            '-rpcuser=%s' % user, '-stdinrpcpass', input=password).getblockcount())
+            '-rpcuser={}'.format(user), '-stdinrpcpass', input=password).getblockcount())
         assert_raises_process_error(1, "incorrect rpcuser or rpcpassword", self.nodes[0].cli(
-            '-rpcuser=%s' % user, '-stdinrpcpass', input="foo").echo)
+            '-rpcuser={}'.format(user), '-stdinrpcpass', input="foo").echo)
 
         self.log.info("Test -stdin and -stdinrpcpass")
-        assert_equal(["foo", "bar"], self.nodes[0].cli('-rpcuser=%s' % user,
+        assert_equal(["foo", "bar"], self.nodes[0].cli('-rpcuser={}'.format(user),
                                                        '-stdin', '-stdinrpcpass', input=password + "\nfoo\nbar").echo())
         assert_raises_process_error(1, "incorrect rpcuser or rpcpassword", self.nodes[0].cli(
-            '-rpcuser=%s' % user, '-stdin', '-stdinrpcpass', input="foo").echo)
+            '-rpcuser={}'.format(user), '-stdin', '-stdinrpcpass', input="foo").echo)
 
 
 if __name__ == '__main__':
diff --git a/test/functional/interface_zmq.py b/test/functional/interface_zmq.py
--- a/test/functional/interface_zmq.py
+++ b/test/functional/interface_zmq.py
@@ -44,8 +44,10 @@
         self.zmqSubSocket.setsockopt(zmq.SUBSCRIBE, b"rawtx")
         ip_address = "tcp://127.0.0.1:28332"
         self.zmqSubSocket.connect(ip_address)
-        self.extra_args = [['-zmqpubhashblock=%s' % ip_address, '-zmqpubhashtx=%s' % ip_address,
-                            '-zmqpubrawblock=%s' % ip_address, '-zmqpubrawtx=%s' % ip_address], []]
+        self.extra_args = [['-zmqpubhashblock={}'.format(ip_address),
+                            '-zmqpubhashtx={}'.format(ip_address),
+                            '-zmqpubrawblock={}'.format(ip_address),
+                            '-zmqpubrawtx={}'.format(ip_address)], []]
         self.add_nodes(self.num_nodes, self.extra_args)
         self.start_nodes()