Make render_expression use the stack like mock_engine.

sqlalchemy_utils/functions/__init__.py

@@ -340,7 +340,7 @@ def mock_engine(engine, stream=None):
     six.exec_('del __mock', frame.f_globals, frame.f_locals)
 
 
-def render_expression(expression, bind, context=None):
+def render_expression(expression, bind, stream=None):
     """Generate a SQL expression from the passed python expression.
 
     Only the global variable, `engine`, is available for use in the
@@ -351,15 +351,36 @@ def render_expression(expression, bind, context=None):
     blindly pass user input to this function as it uses exec.
 
     :param bind: A SQLAlchemy engine or bind URL.
-    :param context: Dictionary of local variables for the expression.
+    :param stream: Render all DDL operations to the stream.
     """
 
+    # Create a stream if not present.
+
+    if stream is None:
         stream = cStringIO()
+
     engine = create_mock_engine(bind, stream)
 
-    six.exec_(expression, {'engine': engine}, context)
+    # Navigate the stack and find the calling frame that allows the
+    # expression to execuate.
 
-    return stream.getvalue()
+    for frame in inspect.stack()[1:]:
+
+        try:
+            frame = frame[0]
+            local = dict(frame.f_locals)
+            local['engine'] = engine
+            six.exec_(expression, frame.f_globals, local)
+            break
+
+        except:
+            pass
+
+    else:
+
+        raise ValueError('Not a valid python expression', engine)
+
+    return stream
 
 
 def render_statement(statement, bind=None):

tests/test_utility_functions.py

@@ -102,9 +102,10 @@ class TestRender(TestCase):
         assert 'WHERE user.id = 3' in text
 
     def test_render_ddl(self):
-        context = {'table': self.User.__table__}
-        expression = 'table.create(engine)'
-        text = render_expression(expression, self.engine, context)
+        expression = 'self.User.__table__.create(engine)'
+        stream = render_expression(expression, self.engine)
+
+        text = stream.getvalue()
 
         assert 'CREATE TABLE user' in text
         assert 'PRIMARY KEY' in text