opf: Fix get_shadows for no branches

Author: coroa

pypsa/opf.py

@@ -1167,6 +1167,8 @@ def set_from_series(df, series):
         df.loc[snapshots] = series.unstack(0).reindex(columns=df.columns)
 
     def get_shadows(constraint, multiind=True):
+        if len(constraint) == 0: return pd.Series()
+
         index = list(constraint.keys())
         if multiind:
             index = pd.MultiIndex.from_tuples(index)

test/test_unit_commitment.py

@@ -134,3 +134,8 @@ def test_minimum_down_time():
     expected_dispatch = np.array([[3000,0,0,8000],[0,800,3000,0]],dtype=float).T
 
     np.testing.assert_array_almost_equal(nu.generators_t.p.values,expected_dispatch)
+
+if __name__ == "__main__":
+    test_minimum_down_time()
+    test_minimum_up_time()
+    test_part_load()