diff --git a/algorithms/automata/dfa.py b/algorithms/automata/dfa.py
index 36211c7e8..34f73a4b4 100644
--- a/algorithms/automata/dfa.py
+++ b/algorithms/automata/dfa.py
@@ -3,16 +3,15 @@ def DFA(transitions, start, final, string):
     num = len(string)
     num_final = len(final)
     cur = start
-    
-    for i in range (num):
-        
-        if transitions[cur][string[i]] == None:
+
+    for i in range(num):
+
+        if transitions[cur][string[i]] is None:
             return False
         else:
             cur = transitions[cur][string[i]]
 
-    for i in range (num_final):
+    for i in range(num_final):
         if cur == final[i]:
             return True
     return False
-        
diff --git a/algorithms/graph/tarjan.py b/algorithms/graph/tarjan.py
index a2f44e58d..399a39d6d 100644
--- a/algorithms/graph/tarjan.py
+++ b/algorithms/graph/tarjan.py
@@ -5,6 +5,7 @@
 """
 from algorithms.graph.graph import DirectedGraph
 
+
 class Tarjan(object):
     def __init__(self, dict_graph):
         self.graph = DirectedGraph(dict_graph)
@@ -18,7 +19,7 @@ def __init__(self, dict_graph):
 
         self.sccs = []
         for v in self.graph.nodes:
-            if v.index == None:
+            if v.index is None:
                 self.strongconnect(v, self.sccs)
 
     def strongconnect(self, v, sccs):
@@ -31,7 +32,7 @@ def strongconnect(self, v, sccs):
 
         # Consider successors of v
         for w in self.graph.adjmt[v]:
-            if w.index == None:
+            if w.index is None:
                 # Successor w has not yet been visited; recurse on it
                 self.strongconnect(w, sccs)
                 v.lowlink = min(v.lowlink, w.lowlink)
@@ -41,7 +42,7 @@ def strongconnect(self, v, sccs):
                 # Note: The next line may look odd - but is correct.
                 # It says w.index not w.lowlink; that is deliberate and from the original paper
                 v.lowlink = min(v.lowlink, w.index)
-        
+
         # If v is a root node, pop the stack and generate an SCC
         if v.lowlink == v.index:
             # start a new strongly connected component
@@ -54,4 +55,3 @@ def strongconnect(self, v, sccs):
                     break
             scc.sort()
             sccs.append(scc)
-    
diff --git a/algorithms/strings/multiply_strings.py b/algorithms/strings/multiply_strings.py
index 0ddc3abf4..2cb0d2206 100644
--- a/algorithms/strings/multiply_strings.py
+++ b/algorithms/strings/multiply_strings.py
@@ -12,8 +12,7 @@
 """
 
 
-def multiply(num1:"str", num2:"str")->"str":
-    carry = 1
+def multiply(num1: "str", num2: "str") -> "str":
     interm = []
     zero = ord('0')
     i_pos = 1