Wrap the WHEN expression of a trigger between parenthesis, as require…

…d by syntax

The drawback is that the BoolExpr appears even more ugly now, within three (!) parens... Sooner
or later I will try to find a solution, but the problem is not easy.

This fixes issue #18.

pglast/printers/ddl.py

@@ -1144,9 +1144,10 @@ def create_trig_stmt(node, output):
         output.newline()
         with output.push_indent(2):
             if node.whenClause:
-                output.write('WHEN ')
+                output.write('WHEN (')
                 with output.expression():
                     output.print_node(node.whenClause)
+                output.write(')')
                 output.newline()
 
             output.write('EXECUTE PROCEDURE ')

tests/test_printers_prettification/ddl/create_trigger.sql

@@ -27,16 +27,16 @@ CREATE TRIGGER trg_celery_periodic_tasks_950_after_update
   AFTER UPDATE
   ON celery.periodic_tasks
   FOR EACH ROW
-    WHEN ((   old.active IS DISTINCT FROM new.active
-           OR old.args IS DISTINCT FROM new.args
-           OR old.kwargs IS DISTINCT FROM new.kwargs
-           OR old.options IS DISTINCT FROM new.options
-           OR old.run_every IS DISTINCT FROM new.run_every
-           OR old.run_at_minutes IS DISTINCT FROM new.run_at_minutes
-           OR old.run_at_hours IS DISTINCT FROM new.run_at_hours
-           OR old.run_on_weekdays IS DISTINCT FROM new.run_on_weekdays
-           OR old.run_on_days IS DISTINCT FROM new.run_on_days
-           OR old.run_on_months IS DISTINCT FROM new.run_on_months))
+    WHEN (((   old.active IS DISTINCT FROM new.active
+            OR old.args IS DISTINCT FROM new.args
+            OR old.kwargs IS DISTINCT FROM new.kwargs
+            OR old.options IS DISTINCT FROM new.options
+            OR old.run_every IS DISTINCT FROM new.run_every
+            OR old.run_at_minutes IS DISTINCT FROM new.run_at_minutes
+            OR old.run_at_hours IS DISTINCT FROM new.run_at_hours
+            OR old.run_on_weekdays IS DISTINCT FROM new.run_on_weekdays
+            OR old.run_on_days IS DISTINCT FROM new.run_on_days
+            OR old.run_on_months IS DISTINCT FROM new.run_on_months)))
     EXECUTE PROCEDURE celery.notify_periodic_tasks_changed()
 
 CREATE TRIGGER trig1 AFTER UPDATE ON schema1.table1