Add a comment in the compiler on why the NOT IN operator

is rendered using brackets.

Change-Id: I39c2e6a03e8aad7ff27bc6000da5bc9f1de14dc2

1 files changed, 3 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 437a11a60..dedd75f5c 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -1908,6 +1908,9 @@ class SQLCompiler(Compiled):
         return self._render_in_expr_w_bindparam(binary, operator, **kw)
 
     def visit_not_in_op_binary(self, binary, operator, **kw):
+        # The brackets are required in the NOT IN operation because the empty
+        # case is handled using the form "(col NOT IN (null) OR 1 = 1)".
+        # The presence of the OR makes the brackets required.
         return "(%s)" % self._render_in_expr_w_bindparam(
             binary, operator, **kw
         )