Support readonly and deferrable for non-serializable transactions (#747)

Resolves #743

Author: pauldraper

asyncpg/transaction.py

@@ -46,17 +46,6 @@ def __init__(self, connection, isolation, readonly, deferrable):
  'isolation is expected to be either of {}, '
  'got {!r}'.format(ISOLATION_LEVELS, isolation))
 
- if isolation and isolation != 'serializable':
- if readonly:
- raise ValueError(
- '"readonly" is only supported for '
- 'serializable transactions')
-
- if deferrable and not readonly:
- raise ValueError(
- '"deferrable" is only supported for '
- 'serializable readonly transactions')
-
  self._isolation = isolation
  self._readonly = readonly
  self._deferrable = deferrable
@@ -132,19 +121,18 @@ async def start(self):
  self._id = con._get_unique_id('savepoint')
  query = 'SAVEPOINT {};'.format(self._id)
  else:
- if self._isolation is None:
- query = 'BEGIN;'
- elif self._isolation == 'read_committed':
- query = 'BEGIN ISOLATION LEVEL READ COMMITTED;'
+ query = 'BEGIN'
+ if self._isolation == 'read_committed':
+ query += ' ISOLATION LEVEL READ COMMITTED'
  elif self._isolation == 'repeatable_read':
- query = 'BEGIN ISOLATION LEVEL REPEATABLE READ;'
- else:
- query = 'BEGIN ISOLATION LEVEL SERIALIZABLE'
-  if self._readonly:
-  query += ' READ ONLY'
-  if self._deferrable:
-  query += ' DEFERRABLE'
-  query += ';'
+ query += ' ISOLATION LEVEL REPEATABLE READ'
+ elif self._isolation == 'serializable':
+ query += ' ISOLATION LEVEL SERIALIZABLE'
+ if self._readonly:
+ query += ' READ ONLY'
+ if self._deferrable:
+ query += ' DEFERRABLE'
+ query += ';'
 
  try:
  await self._connection.execute(query)