Add simple test for adding Nodes and Flows

tests/test_energy_system.py

@@ -14,13 +14,34 @@
 """
 
 from oemof.network import energy_system as es
+from oemof.network.network import Edge
 from oemof.network.network.nodes import Node
 
 
 class TestsEnergySystem:
     def setup_method(self):
         self.es = es.EnergySystem()
 
+    def test_add_nodes(self):
+        assert not self.es.nodes
+
+        node1 = Node(label="node1")
+        self.es.add(node1)
+        assert self.es.nodes
+        assert node1 in self.es.nodes
+        assert not self.es.flows()
+
+        # Note that node2 is not added, but the Flow is already
+        # registred. We do not assert the latter fact as this is not a
+        # guaranteed functionality.
+        node2 = Node(label="node2", inputs={node1: Edge()})
+        assert node2 not in self.es.nodes
+
+        # When both nodes are registred, also the Flow needs to be there.
+        self.es.add(node2)
+        assert node2 in self.es.nodes
+        assert (node1, node2) in self.es.flows().keys()
+
     def test_that_node_additions_are_signalled(self):
         """
         When a node gets `add`ed, a corresponding signal should be emitted.