Work around a random-bug in Python 3.5. See the comment for details.

cfgrib/xarray_to_grib.py

@@ -47,6 +47,14 @@
     'polar_stereographic', 'reduced_gg', 'reduced_ll', 'regular_gg', 'regular_ll', 'rotated_gg',
     'rotated_ll', 'sh',
 ]
+MESSAGE_DEFINITION_KEYS = [
+    # for the GRIB 2 sample we must set this before setting 'totalNumber'
+    'productDefinitionTemplateNumber',
+    # NO IDEA WHAT IS GOING ON HERE: saving regular_ll_msl.grib results in the wrong `paramId`
+    #   unless `units` is set before some other unknown key, this happens at random and only in
+    #   Python 3.5, so it must be linked to dict key stability.
+    'units',
+]
 
 
 def regular_ll_params(values, min_value=-180., max_value=360.):
@@ -172,6 +180,11 @@ def make_template_message(merged_grib_keys, template_path=None, sample_name=None
             sample_name = detect_sample_name(merged_grib_keys)
         template_message = cfgrib.CfMessage.from_sample_name(sample_name)
 
+    for key in MESSAGE_DEFINITION_KEYS:
+        if key in list(merged_grib_keys):
+            template_message[key] = merged_grib_keys[key]
+            merged_grib_keys.pop(key)
+
     for key, value in merged_grib_keys.items():
         try:
             template_message[key] = value