return empty dataframe instead of none (#75)

* return empty dataframe instead of none

* updates history.rst

HISTORY.rst

@@ -15,6 +15,7 @@ UNRELEASED
 * drop `Tijdstip` column in `ddlpy.measurements()` output dataframe to avoid duplication with time index in https://github.com/deltares/ddlpy/pull/52 and https://github.com/deltares/ddlpy/pull/54
 * add `ddlpy.measurements_amount()` to retrieve the number of available measurements grouped by day/month/year in https://github.com/Deltares/ddlpy/pull/63
 * catch accidentally switched start/end dates in https://github.com/Deltares/ddlpy/pull/65
+* in case of no measurements, return empty dataframe instead of None or empty list in https://github.com/Deltares/ddlpy/pull/75
 
 0.1.0 (2019-01-03)
 ------------------

ddlpy/ddlpy.py

@@ -330,9 +330,9 @@ def measurements(location, start_date, end_date, clean_df=True):
             continue
 
     if len(measurements) == 0:
-        # early return in case of no data
+        # return empty dataframe in case of no data
         logger.debug("no data found for this station and time extent")
-        return
+        return pd.DataFrame()
 
     measurements = pd.concat(measurements)
 

tests/test_ddlpy.py

@@ -46,6 +46,14 @@ def test_measurements_amount(location):
     assert data_amount_jaar["Groeperingsperiode"].str.len().iloc[0] == 4
 
 
+def test_measurements_empty(location):
+    """measurements for a location """
+    start_date = dt.datetime(2153, 1, 1)
+    end_date = dt.datetime(2153, 1, 2)
+    measurements = ddlpy.measurements(location, start_date=start_date, end_date=end_date)
+    assert measurements.empty
+
+
 def test_measurements(location):
     """measurements for a location """
     start_date = dt.datetime(1953, 1, 1)