- calling expr.in_([]), i.e. with an empty list, emits a warning

before issuing the usual "expr != expr" clause.  The
"expr != expr" can be very expensive, and it's preferred
that the user not issue in_() if the list is empty,
instead simply not querying, or modifying the criterion
as appropriate for more complex situations.
[ticket:1628]

1 files changed, 9 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/expression.py b/lib/sqlalchemy/sql/expression.py
index 2f0ac90af..acfb3d7c4 100644
--- a/lib/sqlalchemy/sql/expression.py
+++ b/lib/sqlalchemy/sql/expression.py
@@ -1498,7 +1498,15 @@ class _CompareMixin(ColumnOperators):
             args.append(o)
 
         if len(args) == 0:
-            # Special case handling for empty IN's, behave like comparison against zero row selectable
+            # Special case handling for empty IN's, behave like comparison 
+            # against zero row selectable.  We use != to build the 
+            # contradiction as it handles NULL values appropriately, i.e.
+            # "not (x IN ())" should not return NULL values for x.
+            util.warn("The IN-predicate on \"%s\" was invoked with an empty sequence. "
+                       "This results in a contradiction, which nonetheless can be "
+                       "expensive to evaluate.  Consider alternative strategies for "
+                       "improved performance." % self)
+                       
             return self != self
 
         return self.__compare(op, ClauseList(*args).self_group(against=op), negate=negate_op)