fully support isolation_level parameter in base dialect

Generalized the :paramref:`_sa.create_engine.isolation_level` parameter to
the base dialect so that it is no longer dependent on individual dialects
to be present. This parameter sets up the "isolation level" setting to
occur for all new database connections as soon as they are created by the
connection pool, where the value then stays set without being reset on
every checkin.

The :paramref:`_sa.create_engine.isolation_level` parameter is essentially
equivalent in functionality to using the
:paramref:`_engine.Engine.execution_options.isolation_level` parameter via
:meth:`_engine.Engine.execution_options` for an engine-wide setting. The
difference is in that the former setting assigns the isolation level just
once when a connection is created, the latter sets and resets the given
level on each connection checkout.

Fixes: #6342
Change-Id: Id81d6b1c1a94371d901ada728a610696e09e9741

1 files changed, 47 insertions, 0 deletions

diff --git a/lib/sqlalchemy/engine/interfaces.py b/lib/sqlalchemy/engine/interfaces.py
index 38d2c7a57..fbdf9b829 100644
--- a/lib/sqlalchemy/engine/interfaces.py
+++ b/lib/sqlalchemy/engine/interfaces.py
@@ -919,6 +919,11 @@ class Dialect(object):
         isolation level facilities; these APIs should be preferred for
         most typical use cases.
 
+        If the dialect also implements the
+        :meth:`.Dialect.get_isolation_level_values` method, then the given
+        level is guaranteed to be one of the string names within that sequence,
+        and the method will not need to anticipate a lookup failure.
+
         .. seealso::
 
             :meth:`_engine.Connection.get_isolation_level`
@@ -990,6 +995,48 @@ class Dialect(object):
         """
         raise NotImplementedError()
 
+    def get_isolation_level_values(self, dbapi_conn):
+        """return a sequence of string isolation level names that are accepted
+        by this dialect.
+
+        The available names should use the following conventions:
+
+        * use UPPERCASE names.   isolation level methods will accept lowercase
+          names but these are normalized into UPPERCASE before being passed
+          along to the dialect.
+        * separate words should be separated by spaces, not underscores, e.g.
+          ``REPEATABLE READ``.  isolation level names will have underscores
+          converted to spaces before being passed along to the dialect.
+        * The names for the four standard isolation names to the extent that
+          they are supported by the backend should be ``READ UNCOMMITTED``
+          ``READ COMMITTED``, ``REPEATABLE READ``, ``SERIALIZABLE``
+        * if the dialect supports an autocommit option it should be provided
+          using the isolation level name ``AUTOCOMMIT``.
+        * Other isolation modes may also be present, provided that they
+          are named in UPPERCASE and use spaces not underscores.
+
+        This function is used so that the default dialect can check that
+        a given isolation level parameter is valid, else raises an
+        :class:`_exc.ArgumentError`.
+
+        A DBAPI connection is passed to the method, in the unlikely event that
+        the dialect needs to interrogate the connection itself to determine
+        this list, however it is expected that most backends will return
+        a hardcoded list of values.  If the dialect supports "AUTOCOMMIT",
+        that value should also be present in the sequence returned.
+
+        The method raises ``NotImplementedError`` by default.  If a dialect
+        does not implement this method, then the default dialect will not
+        perform any checking on a given isolation level value before passing
+        it onto the :meth:`.Dialect.set_isolation_level` method.  This is
+        to allow backwards-compatibility with third party dialects that may
+        not yet be implementing this method.
+
+        .. versionadded:: 2.0
+
+        """
+        raise NotImplementedError()
+
     @classmethod
     def get_dialect_cls(cls, url):
         """Given a URL, return the :class:`.Dialect` that will be used.