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Author: marqh

lib/iris/fileformats/grib/_load_convert.py

@@ -158,13 +158,21 @@ def _unscale(v, f):
 
     if isinstance(value, Iterable) or isinstance(factor, Iterable):
         def _masker(item):
+            # This is a small work around for an edge case, which is not
+            # evident in any of our sample GRIB2 messages, where an array
+            # of header elements contains missing values.
+            # iris.fileformats.grib.message returns these as None, but they
+            # are wanted as a numerical masked array, so a temporary mdi
+            # value is used, selected from a legacy implementation of iris,
+            # to construct the masked array. The valure is transient, only in
+            # scope for this function.
             numerical_mdi = 2 ** 32 - 1
             item = [numerical_mdi if i is None else i for i in item]
             result = ma.masked_equal(item, numerical_mdi)
             if ma.count_masked(result):
                 # Circumvent downstream NumPy "RuntimeWarning"
                 # of "overflow encountered in power" in _unscale
-                # for data containing _MDI.
+                # for data containing _MDI.  Remove transient _MDI value.
                 result.data[result.mask] = 0
             return result
         value = _masker(value)

lib/iris/fileformats/grib/message.py

@@ -458,13 +458,9 @@ def _get_key_value(self, key):
         elif key in ('typeOfFirstFixedSurface', 'typeOfSecondFixedSurface'):
             # By default these values are returned as unhelpful strings but
             # we can use int representation to compare against instead.
-            res = gribapi.grib_get(self._message_id, key, int)
-            if gribapi.grib_is_missing(self._message_id, key) == 1:
-                res = None
+            res = _get_value_or_missing(self._message_id, key)
         else:
-            res = gribapi.grib_get(self._message_id, key)
-            if gribapi.grib_is_missing(self._message_id, key) == 1:
-                res = None
+            res = _get_value_or_missing(self._message_id, key)
         return res
 
     def get_computed_key(self, key):
@@ -485,11 +481,21 @@ def get_computed_key(self, key):
         if key in vector_keys:
             res = gribapi.grib_get_array(self._message_id, key)
         else:
-            res = gribapi.grib_get(self._message_id, key)
-            if gribapi.grib_is_missing(self._message_id, key) == 1:
-                res = None
+            res = _get_value_or_missing(self._message_id, key)
         return res
 
     def keys(self):
         """Return coded keys available in this Section."""
         return self._keys
+
+    def _get_value_or_missing(message_id, key):
+        """
+        Return value of header element, or None if value is encoded as missing.
+        Implementation of Regulations 92.1.4 and 92.1.5 via ECCodes.
+
+        """
+        if gribapi.grib_is_missing(self._message_id, key) == 1:
+            res = None
+        else:
+            res = gribapi.grib_get(self._message_id, key)
+        return result