Try to avoid some locking issues.

dataset/persistence/database.py

@@ -46,9 +46,15 @@ class Database(object):
     def _release(self):
         if not hasattr(self.local, 'tx'):
             self.lock.release()
+            self.local.must_release = False
         else:
             self.local.must_release = True
 
+    def _release_internal(self):
+        if not hasattr(self.local, 'must_release') and self.local.must_release:
+            self.lock.release()
+            self.local.must_release = False
+
     def begin(self):
         """ Enter a transaction explicitly. No data will be written
         until the transaction has been committed. """
@@ -62,18 +68,14 @@ class Database(object):
         since the transaction was begun permanent. """
         self.local.tx.commit()
         del self.local.tx
-        if not hasattr(self.local, 'must_release'):
-            self.lock.release()
-            del self.local.must_release
+        self._release_internal()
 
     def rollback(self):
         """ Roll back the current transaction, discarding all statements
         executed since the transaction was begun. """
         self.local.tx.rollback()
         del self.local.tx
-        if not hasattr(self.local, 'must_release'):
-            self.lock.release()
-            del self.local.must_release
+        self._release_internal()
 
     @property
     def tables(self):

dataset/persistence/table.py

@@ -213,7 +213,7 @@ class Table(object):
             if name not in self.table.columns.keys():
                 col = Column(name, type)
                 col.create(self.table,
-                       connection=self.database.engine)
+                           connection=self.database.executable)
         finally:
             self.database._release()
 

setup.py

@@ -2,7 +2,7 @@ from setuptools import setup, find_packages
 
 setup(
     name='dataset',
-    version='0.3.5',
+    version='0.3.6',
     description="Toolkit for Python-based data processing.",
     long_description="",
     classifiers=[