Add "impl" parameter to PickleType

Add a impl parameter to :class:`_types.PickleType` constructor, allowing
any arbitary type to be used in place of the default implementation of
:class:`_types.LargeBinary`. Pull request courtesy jason3gb.

Fixes: #6646
Closes: #6657
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/6657
Pull-request-sha: e49bcd368d1f71dba92225d8d6e3af2bbe7142f7

Change-Id: Ib79f3b0ebbc94393f673f5a5ba6558260083d0cf

1 files changed, 17 insertions, 0 deletions

diff --git a/test/sql/test_types.py b/test/sql/test_types.py
index 9db0fee3b..3cbd2c07f 100644
--- a/test/sql/test_types.py
+++ b/test/sql/test_types.py
@@ -3809,6 +3809,23 @@ class PickleTest(fixtures.TestBase):
         ):
             assert p1.compare_values(p1.copy_value(obj), obj)
 
+    @testing.combinations(
+        None, mysql.LONGBLOB, LargeBinary, mysql.LONGBLOB(), LargeBinary()
+    )
+    def test_customized_impl(self, impl):
+        """test #6646"""
+
+        if impl is None:
+            p1 = PickleType()
+            assert isinstance(p1.impl, LargeBinary)
+        else:
+            p1 = PickleType(impl=impl)
+
+            if not isinstance(impl, type):
+                impl = type(impl)
+
+            assert isinstance(p1.impl, impl)
+
 
 class CallableTest(fixtures.TestBase):
     @testing.provide_metadata