Add test for linear_extra_interpolation()

Author: SabineHaas

tests/test_tools.py

@@ -1,5 +1,6 @@
-from windpowerlib.tools import smallest_difference
+from windpowerlib.tools import smallest_difference, linear_extra_interpolation
 import pandas as pd
+from pandas.util.testing import assert_series_equal
 
 
 class TestTools:
@@ -27,3 +28,32 @@ def test_smallest_difference(self):
         expected_output = (100, 4.0)
         parameters['comp_value'] = 90
         assert smallest_difference(**parameters) == expected_output
+
+    def test_linear_extra_interpolation(self):
+        weather = pd.DataFrame(data={'v_wind': [4.0, 5.0, 6.0]},
+                               index=[100, 150, 200])
+        weather_pd_series = pd.DataFrame(data={'v_wind': [
+            pd.Series(data=[4.0, 5.0, 6.0]),
+            pd.Series(data=[8.0, 10.0, 14.0]),
+            pd.Series(data=[16.0, 20.0, 28.0])]}, index=[100, 150, 200])
+        # TODO: test v_wind as np.array
+
+        # Entries in column v_wind are float
+        expected_output = 4.0
+        assert (linear_extra_interpolation(weather, 100, 'v_wind') ==
+                expected_output)
+        expected_output = 5.5
+        assert (linear_extra_interpolation(weather, 175, 'v_wind') ==
+                expected_output)
+        expected_output = 7.0
+        assert (linear_extra_interpolation(weather, 250, 'v_wind') ==
+                expected_output)
+        expected_output = 3.0
+        assert (linear_extra_interpolation(weather, 50, 'v_wind') ==
+                expected_output)
+
+        # Entries in column v_wind are pd.Series
+        expected_output = pd.Series(data=[4.0, 5.0, 6.0])
+        assert_series_equal(linear_extra_interpolation(weather_pd_series,
+                                                       100, 'v_wind'),
+                            expected_output)