Fixed valuecount for cases where all pixels are nodata.

Author: yellowcap

raster/valuecount.py

@@ -170,9 +170,14 @@ def statistics(self, reset=False):
             for data in self.tiles():
                 self._push_stats(data)
 
-        # Compute mean and std from totals sums
-        mean = self._stats_t1 / self._stats_t0
-        std = numpy.sqrt(self._stats_t0 * self._stats_t2 - self._stats_t1 * self._stats_t1) / self._stats_t0
+        if self._stats_t0 == 0:
+            # If totals sum is zero, no data was available to comput statistics
+            mean = None
+            std = None
+        else:
+            # Compute mean and std from totals sums.
+            mean = self._stats_t1 / self._stats_t0
+            std = numpy.sqrt(self._stats_t0 * self._stats_t2 - self._stats_t1 * self._stats_t1) / self._stats_t0
 
         return (self._stats_min_value, self._stats_max_value, mean, std)
 

tests/test_valuecount.py

@@ -249,3 +249,17 @@ def test_valuecount_exception(self):
 
     def test_valuecount_pixelsize(self):
         self.assertAlmostEqual(self.rasterlayer.pixelsize()[0], tile_scale(11))
+
+    def test_full_mask_data(self):
+        # Override all tiles to be fully masked.
+        for tile in self.rasterlayer.rastertile_set.all():
+            tile.rast.bands[0].data([0], shape=(1, 1))
+            tile.rast.bands[0].nodata_value = 0
+            tile.save()
+
+        # Use a legend with simple int expression
+        agg = Aggregator(
+            layer_dict={'a': self.rasterlayer.id},
+            formula='a',
+        )
+        self.assertEqual((None, None, None, None), agg.statistics())