transaction: Fix error messages; test that __aenter__ returns None

Author: 1st1

asyncpg/transaction.py

@@ -48,7 +48,7 @@ def __init__(self, connection, isolation, readonly, deferrable):
     async def __aenter__(self):
         if self._managed:
             raise apg_errors.InterfaceError(
-                'cannot enter context: already in an `async def` block')
+                'cannot enter context: already in an `async with` block')
         self._managed = True
         await self.start()
 
@@ -168,13 +168,13 @@ async def __rollback(self):
     async def commit(self):
         if self._managed:
             raise apg_errors.InterfaceError(
-                'cannot manually commit from within an `async def` block')
+                'cannot manually commit from within an `async with` block')
         await self.__commit()
 
     async def rollback(self):
         if self._managed:
             raise apg_errors.InterfaceError(
-                'cannot manually rollback from within an `async def` block')
+                'cannot manually rollback from within an `async with` block')
         await self.__rollback()
 
     def __repr__(self):

tests/test_transaction.py

@@ -11,9 +11,14 @@ async def test_transaction_regular(self):
         self.assertIsNone(self.con._top_xact)
 
         with self.assertRaises(ZeroDivisionError):
-            async with tr:
+            async with tr as with_tr:
                 self.assertIs(self.con._top_xact, tr)
 
+                # We don't return the transaction object from __aenter__,
+                # to make it harder for people to use '.rollback()' and
+                # '.commit()' from within an 'async with' block.
+                self.assertIsNone(with_tr)
+
                 await self.con.execute('''
                     CREATE TABLE mytab (a int);
                 ''')
@@ -107,23 +112,23 @@ async def test_transaction_interface_errors(self):
 
         tr = self.con.transaction()
         with self.assertRaisesRegex(asyncpg.InterfaceError,
-                                    'cannot manually commit.*async def'):
+                                    'cannot manually commit.*async with'):
             async with tr:
                 await tr.commit()
 
         self.assertIsNone(self.con._top_xact)
 
         tr = self.con.transaction()
         with self.assertRaisesRegex(asyncpg.InterfaceError,
-                                    'cannot manually rollback.*async def'):
+                                    'cannot manually rollback.*async with'):
             async with tr:
                 await tr.rollback()
 
         self.assertIsNone(self.con._top_xact)
 
         tr = self.con.transaction()
         with self.assertRaisesRegex(asyncpg.InterfaceError,
-                                    'cannot enter context'):
+                                    'cannot enter context:.*async with'):
             async with tr:
                 async with tr:
                     pass