diff --git a/neet/boolean/random/topology.py b/neet/boolean/random/topology.py
index 27f7e88..56daf92 100644
--- a/neet/boolean/random/topology.py
+++ b/neet/boolean/random/topology.py
@@ -20,8 +20,8 @@ def constraints(self, constraints):
         Set the randomizer's constraints.
 
         :param constraints: the new constraints
-        :type constraints: a seq of AbstractConstraint instances
-        :raises TypeError: if any of the contraints are not an AbstractConstraint
+        :type constraints: a seq of AbstractConstraint or callable instances
+        :raises TypeError: if any of the contraints are neither an AbstractConstraint nor callable
         """
         if constraints is None:
             constraints = []
diff --git a/test/boolean/random/test_topology.py b/test/boolean/random/test_topology.py
new file mode 100644
index 0000000..72b3df8
--- /dev/null
+++ b/test/boolean/random/test_topology.py
@@ -0,0 +1,112 @@
+import unittest
+import networkx as nx
+import numpy as np
+from itertools import islice
+from neet.boolean.random import topology
+from neet.boolean.random.constraints import ConstraintError, HasExternalNodes
+
+
+class TestTopology(unittest.TestCase):
+    def test_fixed_topology_without_constraints(self):
+        for net in self.erdos_renyi(10, 10, 0.5, directed=True):
+            gen = topology.FixedTopology(net)
+            for r in islice(gen, 10):
+                self.assertTrue(nx.is_isomorphic(net, r))
+
+    def test_fixed_topology_with_init_constraints(self):
+        for net in self.erdos_renyi(10, 10, 0.5, directed=True):
+            num_external = np.count_nonzero([d == 0 for _, d in net.in_degree()])
+            gen = topology.FixedTopology(net, constraints=[HasExternalNodes(num_external)])
+            for r in islice(gen, 10):
+                self.assertTrue(nx.is_isomorphic(net, r))
+
+            with self.assertRaises(ConstraintError):
+                topology.FixedTopology(net, constraints=[HasExternalNodes(num_external + 1)])
+
+            gen = topology.FixedTopology(net, constraints=[lambda net: len(net) == 10])
+            for r in islice(gen, 10):
+                self.assertTrue(nx.is_isomorphic(net, r))
+
+            with self.assertRaises(ConstraintError):
+                topology.FixedTopology(net, constraints=[lambda net: len(net) != 10])
+
+    def test_fixed_topology_with_added_constraints(self):
+        for net in self.erdos_renyi(10, 10, 0.5, directed=True):
+            num_external = np.count_nonzero([d == 0 for _, d in net.in_degree()])
+            gen = topology.FixedTopology(net)
+            gen.add_constraint(HasExternalNodes(num_external))
+            self.assertEquals(1, len(gen.constraints))
+            for r in islice(gen, 10):
+                self.assertTrue(nx.is_isomorphic(net, r))
+
+            with self.assertRaises(ConstraintError):
+                gen.add_constraint(HasExternalNodes(num_external + 1))
+            self.assertEquals(1, len(gen.constraints))
+
+            gen.add_constraint(lambda net: len(net) == 10)
+            self.assertEquals(2, len(gen.constraints))
+            for r in islice(gen, 10):
+                self.assertTrue(nx.is_isomorphic(net, r))
+
+            with self.assertRaises(ConstraintError):
+                gen.add_constraint(lambda net: len(net) != 10)
+            self.assertEquals(2, len(gen.constraints))
+
+            with self.assertRaises(TypeError):
+                gen.add_constraint(None)
+            with self.assertRaises(TypeError):
+                gen.add_constraint(1)
+            with self.assertRaises(TypeError):
+                gen.add_constraint(1.5)
+            with self.assertRaises(TypeError):
+                gen.add_constraint(True)
+
+    def test_fixed_topology_with_set_constraints(self):
+        for net in self.erdos_renyi(10, 10, 0.5, directed=True):
+            num_external = np.count_nonzero([d == 0 for _, d in net.in_degree()])
+            gen = topology.FixedTopology(net)
+            gen.constraints = [HasExternalNodes(num_external)]
+            self.assertEquals(1, len(gen.constraints))
+            for r in islice(gen, 10):
+                self.assertTrue(nx.is_isomorphic(net, r))
+
+            with self.assertRaises(ConstraintError):
+                gen.constraints = [HasExternalNodes(num_external + 1)]
+            self.assertEquals(1, len(gen.constraints))
+
+            gen.constraints = [HasExternalNodes(num_external), lambda net: len(net) == 10]
+            self.assertEquals(2, len(gen.constraints))
+            for r in islice(gen, 10):
+                self.assertTrue(nx.is_isomorphic(net, r))
+
+            gen.constraints = []
+            self.assertEquals(0, len(gen.constraints))
+            with self.assertRaises(ConstraintError):
+                gen.constraints = [HasExternalNodes(num_external + 1), lambda net: len(net) != 10]
+            self.assertEquals(0, len(gen.constraints))
+
+            with self.assertRaises(TypeError):
+                gen.constraints = [None]
+            with self.assertRaises(TypeError):
+                gen.constraints = [1]
+            with self.assertRaises(TypeError):
+                gen.constraints = [1.5]
+            with self.assertRaises(TypeError):
+                gen.constraints = [True]
+
+            with self.assertRaises(TypeError):
+                gen.constraints = HasExternalNodes(num_external)
+
+            with self.assertRaises(TypeError):
+                gen.constraints = lambda net: len(net) == 10
+
+    #  def test_mean_degree_without_constraints(self):
+    #      for net in self.erdos_renyi(10, 10, 0.5, directed=True):
+    #          mean_degree = mean
+    #          gen = topology.MeanDegree(net)
+    #          for r in islice(gen, 10):
+    #              neg.degree
+    #              self.assertTrue(nx.is_isomorphic(net, r))
+
+    def erdos_renyi(self, num, *args, **kwargs):
+        return [nx.generators.random_graphs.erdos_renyi_graph(*args, **kwargs) for _ in range(num)]