diff --git a/test/functional/rpc_bind.py b/test/functional/rpc_bind.py
--- a/test/functional/rpc_bind.py
+++ b/test/functional/rpc_bind.py
@@ -26,7 +26,7 @@
         then try to connect, and check if the set of bound addresses
         matches the expected set.
         '''
-        self.log.info("Bind test for %s" % str(addresses))
+        self.log.info("Bind test for {}".format(str(addresses)))
         expected = [(addr_to_hex(addr), port) for (addr, port) in expected]
         base_args = ['-disablewallet', '-nolisten']
         if allow_ips:
@@ -49,7 +49,7 @@
         Start a node with rpcallow IP, and request getnetworkinfo
         at a non-localhost IP.
         '''
-        self.log.info("Allow IP test for %s:%d" % (rpchost, rpcport))
+        self.log.info("Allow IP test for {}:{}".format(rpchost, rpcport))
         base_args = ['-disablewallet', '-nolisten'] + \
             ['-rpcallowip=' + x for x in allow_ips]
         self.nodes[0].host = None
@@ -90,7 +90,7 @@
         except OSError:
             raise SkipTest("This test requires IPv6 support.")
 
-        self.log.info("Using interface %s for testing" % non_loopback_ip)
+        self.log.info("Using interface {} for testing".format(non_loopback_ip))
 
         defaultport = rpc_port(0)
 
diff --git a/test/functional/rpc_invalidateblock.py b/test/functional/rpc_invalidateblock.py
--- a/test/functional/rpc_invalidateblock.py
+++ b/test/functional/rpc_invalidateblock.py
@@ -45,7 +45,7 @@
         newhash = self.nodes[0].getbestblockhash()
         if (newheight != 4 or newhash != besthash):
             raise AssertionError(
-                "Wrong tip for node0, hash %s, height %d" % (newhash, newheight))
+                "Wrong tip for node0, hash {}, height {}".format(newhash, newheight))
 
         self.log.info("\nMake sure we won't reorg to a lower work chain:")
         connect_nodes_bi(self.nodes[1], self.nodes[2])
@@ -67,7 +67,7 @@
         node1height = self.nodes[1].getblockcount()
         if node1height < 4:
             raise AssertionError(
-                "Node 1 reorged to a lower height: %d" % node1height)
+                "Node 1 reorged to a lower height: {}".format(node1height))
 
 
 if __name__ == '__main__':