Handle polynomial scaling with no coefficients (#183)

For issue #182

nptdms/scaling.py

@@ -61,6 +61,9 @@ def from_properties(properties, scale_index):
         return PolynomialScaling(coefficients, input_source)
 
     def scale(self, data):
+        if len(self.coefficients) == 0:
+            return np.zeros(len(data), dtype=np.dtype('float64'))
+
         # Ensure data is double type before scaling
         data = data.astype(np.dtype('float64'), copy=False)
         return np.polynomial.polynomial.polyval(data, self.coefficients)

nptdms/test/test_scaling.py

@@ -72,6 +72,23 @@ def test_polynomial_scaling():
     np.testing.assert_almost_equal(expected_scaled_data, scaled_data)
 
 
+def test_polynomial_scaling_with_no_coefficients():
+    """Test polynomial scaling when there are no coefficients, so data should be all zero
+    """
+    data = StubTdmsData(np.array([1.0, 2.0, 3.0]))
+    expected_scaled_data = np.array([0.0, 0.0, 0.0])
+
+    properties = {
+        "NI_Number_Of_Scales": 1,
+        "NI_Scale[0]_Scale_Type": "Polynomial",
+        "NI_Scale[0]_Polynomial_Coefficients_Size": 0
+    }
+    scaling = get_scaling(properties, {}, {})
+    scaled_data = scaling.scale(data)
+
+    np.testing.assert_almost_equal(expected_scaled_data, scaled_data)
+
+
 def test_polynomial_scaling_with_3_coefficients():
     """Test polynomial scaling"""