Detect unconnected nodes via span generators

Change-Id: I6962b9f193723dc7856b324d5da94d2f46b21c06

tests/test_network_functions.py

@@ -6,6 +6,7 @@
 
 from pathlib import Path
 import pytest
+from gnpy.core.exceptions import NetworkTopologyError
 from gnpy.core.network import span_loss
 from gnpy.tools.json_io import load_equipment, load_network
 
@@ -29,8 +30,6 @@ NETWORK_FILENAME = TEST_DIR / 'data/bugfixiteratortopo.json'
     # fourth span
     ['fiber6', 17],
     ['fused7', 17],
-    # not connected anywhere
-    ['fused4', 1],
     # all other nodes
     ['Site_A', 0],
     ['nodeA', 0],
@@ -50,3 +49,13 @@ def test_span_loss(node, attenuation):
             assert attenuation == span_loss(network, x)
             return
     assert not f'node "{node}" referenced from test but not found in the topology'  # pragma: no cover
+
+
+@pytest.mark.parametrize("node", ['fused4'])
+def test_span_loss_unconnected(node):
+    '''Fused node that has no next and no previous nodes should be detected'''
+    equipment = load_equipment(EQPT_FILENAME)
+    network = load_network(NETWORK_FILENAME, equipment)
+    x = next(x for x in network.nodes() if x.uid == node)
+    with pytest.raises(NetworkTopologyError):
+        span_loss(network, x)