Refactor for improved readability

Double use of the name es also caused an error being hard to detect.

tests/test_energy_system.py

@@ -15,24 +15,24 @@
 
 import pytest
 
-from oemof.network import energy_system as es
+from oemof.network.energy_system import EnergySystem
 from oemof.network.network import Edge
 from oemof.network.network.nodes import Node
 
 
 def test_ensys_init():
     node = Node("label")
-    ensys = es.EnergySystem(nodes=[node])
+    ensys = EnergySystem(nodes=[node])
     assert node in ensys.nodes
 
     with pytest.warns(FutureWarning):
-        ensys = es.EnergySystem(entities=[node])
+        ensys = EnergySystem(entities=[node])
         assert node in ensys.nodes
 
 
 class TestsEnergySystem:
     def setup_method(self):
-        self.es = es.EnergySystem()
+        self.es = EnergySystem()
 
     def test_add_nodes(self):
         assert not self.es.nodes
@@ -54,14 +54,6 @@ def test_add_nodes(self):
         assert node2 in self.es.nodes
         assert (node1, node2) in self.es.flows().keys()
 
-    def test_node_access_warning(self):
-        node_label = "label"
-        self.es.add(Node(node_label))
-        with pytest.warns(
-            match="API to access nodes by label is experimental"
-        ):
-            _ = es.node[node_label]
-
     def test_add_flow_assignment(self):
         assert not self.es.nodes
 
@@ -99,7 +91,7 @@ def subscriber(sender, **kwargs):
 
         subscriber.called = False
 
-        es.EnergySystem.signals[es.EnergySystem.add].connect(
+        EnergySystem.signals[EnergySystem.add].connect(
             subscriber, sender=node
         )
         self.es.add(node)

tests/test_network_classes.py

@@ -19,7 +19,7 @@
 
 import pytest
 
-from oemof.network.energy_system import EnergySystem as EnSys
+from oemof.network.energy_system import EnergySystem
 from oemof.network.network import Bus
 from oemof.network.network import Sink
 from oemof.network.network import Source
@@ -31,7 +31,7 @@
 
 class TestsNode:
     def setup_method(self):
-        self.energysystem = EnSys()
+        self.energysystem = EnergySystem()
 
     def test_entity_initialisation(self):
         entity = Entity(label="foo")
@@ -328,7 +328,7 @@ def test_flow_setter(self):
 
 class TestsEnergySystemNodesIntegration:
     def setup_method(self):
-        self.es = EnSys()
+        self.es = EnergySystem()
 
     def test_entity_registration(self):
         n1 = Node(label="<B1>")
@@ -339,7 +339,10 @@ def test_entity_registration(self):
         assert self.es.node["<B2>"] == n2
         n3 = Node(label="<TF1>", inputs=[n1], outputs=[n2])
         self.es.add(n3)
-        assert self.es.node["<TF1>"] == n3
+        with pytest.warns(
+            match="API to access nodes by label is experimental"
+        ):
+            assert self.es.node["<TF1>"] == n3
 
 
 def test_deprecated_classes():