Patch time_interval for all constant fields

parcels/fieldset.py

@@ -76,7 +76,9 @@ def time_interval(self):
         time_intervals = (f.time_interval for f in self.fields.values())
 
         # Filter out Nones from constant Fields
-        time_intervals = (t for t in time_intervals if t is not None)
+        time_intervals = [t for t in time_intervals if t is not None]
+        if len(time_intervals) == 0:  # All fields are constant fields
+            return None
         return functools.reduce(lambda x, y: x.intersection(y), time_intervals)
 
     def dimrange(self, dim):

tests/v4/test_fieldset.py

@@ -93,6 +93,14 @@ def test_fieldset_time_interval():
     assert fieldset.time_interval.right == np.datetime64("2001-01-01")
 
 
+def test_fieldset_time_interval_constant_fields():
+    fieldset = FieldSet([])
+    fieldset.add_constant_field("constant_field", 1.0)
+    fieldset.add_constant_field("constant_field2", 2.0)
+
+    assert fieldset.time_interval is None
+
+
 def test_fieldset_init_incompatible_calendars():
     ds1 = ds.copy()
     ds1["time"] = xr.date_range("2000", "2001", T_structured, calendar="365_day", use_cftime=True)