diff --git a/csp.py b/csp.py
index d5f96f80b..ee59d4a6b 100644
--- a/csp.py
+++ b/csp.py
@@ -160,7 +160,7 @@ def conflicted_vars(self, current):
 def AC3(csp, queue=None, removals=None):
     """[Figure 6.3]"""
     if queue is None:
-        queue = [(Xi, Xk) for Xi in csp.variables for Xk in csp.neighbors[Xi]]
+        queue = {(Xi, Xk) for Xi in csp.variables for Xk in csp.neighbors[Xi]}
     csp.support_pruning()
     while queue:
         (Xi, Xj) = queue.pop()
@@ -169,7 +169,7 @@ def AC3(csp, queue=None, removals=None):
                 return False
             for Xk in csp.neighbors[Xi]:
                 if Xk != Xj:
-                    queue.append((Xk, Xi))
+                    queue.add((Xk, Xi))
     return True
 
 
@@ -243,7 +243,7 @@ def forward_checking(csp, var, value, assignment, removals):
 
 def mac(csp, var, value, assignment, removals):
     """Maintain arc consistency."""
-    return AC3(csp, [(X, var) for X in csp.neighbors[var]], removals)
+    return AC3(csp, {(X, var) for X in csp.neighbors[var]}, removals)
 
 # The search, proper
 
@@ -374,7 +374,7 @@ def make_arc_consistent(Xj, Xk, csp):
                 # Found a consistent assignment for val1, keep it
                 keep = True
                 break
-        
+
         if not keep:
             # Remove val1
             csp.prune(Xj, val1, None)
diff --git a/tests/test_csp.py b/tests/test_csp.py
index 2bc907b6c..77b35c796 100644
--- a/tests/test_csp.py
+++ b/tests/test_csp.py
@@ -3,7 +3,6 @@
 from csp import *
 import random
 
-
 random.seed("aima-python")
 
 
@@ -174,7 +173,7 @@ def test_csp_conflicted_vars():
 def test_revise():
     neighbors = parse_neighbors('A: B; B: ')
     domains = {'A': [0], 'B': [4]}
-    constraints = lambda X, x, Y, y: x % 2 == 0 and (x+y) == 4
+    constraints = lambda X, x, Y, y: x % 2 == 0 and (x + y) == 4
 
     csp = CSP(variables=None, domains=domains, neighbors=neighbors, constraints=constraints)
     csp.support_pruning()
@@ -196,24 +195,24 @@ def test_revise():
 def test_AC3():
     neighbors = parse_neighbors('A: B; B: ')
     domains = {'A': [0, 1, 2, 3, 4], 'B': [0, 1, 2, 3, 4]}
-    constraints = lambda X, x, Y, y: x % 2 == 0 and (x+y) == 4 and y % 2 != 0
+    constraints = lambda X, x, Y, y: x % 2 == 0 and (x + y) == 4 and y % 2 != 0
     removals = []
 
     csp = CSP(variables=None, domains=domains, neighbors=neighbors, constraints=constraints)
 
     assert AC3(csp, removals=removals) is False
 
-    constraints = lambda X, x, Y, y: (x % 2) == 0 and (x+y) == 4
+    constraints = lambda X, x, Y, y: (x % 2) == 0 and (x + y) == 4
     removals = []
     csp = CSP(variables=None, domains=domains, neighbors=neighbors, constraints=constraints)
 
     assert AC3(csp, removals=removals) is True
     assert (removals == [('A', 1), ('A', 3), ('B', 1), ('B', 3)] or
             removals == [('B', 1), ('B', 3), ('A', 1), ('A', 3)])
-    
-    domains = {'A': [ 2, 4], 'B': [ 3, 5]}
-    constraints = lambda X, x, Y, y: int(x) > int (y)
-    removals=[]
+
+    domains = {'A': [2, 4], 'B': [3, 5]}
+    constraints = lambda X, x, Y, y: int(x) > int(y)
+    removals = []
     csp = CSP(variables=None, domains=domains, neighbors=neighbors, constraints=constraints)
 
     assert AC3(csp, removals=removals)
@@ -247,7 +246,7 @@ def test_num_legal_values():
 def test_mrv():
     neighbors = parse_neighbors('A: B; B: C; C: ')
     domains = {'A': [0, 1, 2, 3, 4], 'B': [4], 'C': [0, 1, 2, 3, 4]}
-    constraints = lambda X, x, Y, y: x % 2 == 0 and (x+y) == 4
+    constraints = lambda X, x, Y, y: x % 2 == 0 and (x + y) == 4
     csp = CSP(variables=None, domains=domains, neighbors=neighbors, constraints=constraints)
     assignment = {'A': 0}
 
@@ -269,13 +268,13 @@ def test_mrv():
 def test_unordered_domain_values():
     map_coloring_test = MapColoringCSP(list('123'), 'A: B C; B: C; C: ')
     assignment = None
-    assert unordered_domain_values('A', assignment,  map_coloring_test) == ['1', '2', '3']
+    assert unordered_domain_values('A', assignment, map_coloring_test) == ['1', '2', '3']
 
 
 def test_lcv():
     neighbors = parse_neighbors('A: B; B: C; C: ')
     domains = {'A': [0, 1, 2, 3, 4], 'B': [0, 1, 2, 3, 4, 5], 'C': [0, 1, 2, 3, 4]}
-    constraints = lambda X, x, Y, y: x % 2 == 0 and (x+y) == 4
+    constraints = lambda X, x, Y, y: x % 2 == 0 and (x + y) == 4
     csp = CSP(variables=None, domains=domains, neighbors=neighbors, constraints=constraints)
     assignment = {'A': 0}
 
@@ -347,7 +346,7 @@ def test_min_conflicts():
     assert min_conflicts(france)
 
     tests = [(usa, None)] * 3
-    assert failure_test(min_conflicts, tests) >= 1/3
+    assert failure_test(min_conflicts, tests) >= 1 / 3
 
     australia_impossible = MapColoringCSP(list('RG'), 'SA: WA NT Q NSW V; NT: WA Q; NSW: Q V; T: ')
     assert min_conflicts(australia_impossible, 1000) is None
@@ -419,9 +418,9 @@ def test_parse_neighbours():
 
 def test_topological_sort():
     root = 'NT'
-    Sort, Parents = topological_sort(australia,root)
+    Sort, Parents = topological_sort(australia, root)
 
-    assert Sort == ['NT','SA','Q','NSW','V','WA']
+    assert Sort == ['NT', 'SA', 'Q', 'NSW', 'V', 'WA']
     assert Parents['NT'] == None
     assert Parents['SA'] == 'NT'
     assert Parents['Q'] == 'SA'
@@ -432,10 +431,11 @@ def test_topological_sort():
 
 def test_tree_csp_solver():
     australia_small = MapColoringCSP(list('RB'),
-                           'NT: WA Q; NSW: Q V')
+                                     'NT: WA Q; NSW: Q V')
     tcs = tree_csp_solver(australia_small)
     assert (tcs['NT'] == 'R' and tcs['WA'] == 'B' and tcs['Q'] == 'B' and tcs['NSW'] == 'R' and tcs['V'] == 'B') or \
            (tcs['NT'] == 'B' and tcs['WA'] == 'R' and tcs['Q'] == 'R' and tcs['NSW'] == 'B' and tcs['V'] == 'R')
 
+
 if __name__ == "__main__":
     pytest.main()