Added an exception for unconfigured databases and cleaned up some comments.

metadata.py

@@ -5,8 +5,6 @@ Everything needed to interact with the metadata database; namely SQL ORM objects
 generator for the actual database, and a handful of convenience functions.
 '''
 import contextlib
-import collections
-import hashlib
 import uuid
 
 import sqlalchemy
@@ -17,9 +15,13 @@ import sqlalchemy.orm
 HASH_CHUNK_SIZE = 10485760 # 10mb
 _db_session_maker = sqlalchemy.orm.sessionmaker()
 _engine = None
+_configured = False
 
 # TODO: Support DAT credit, DAT filenames, and checking DAT completeness.
-DatData = collections.namedtuple('DatData', 'UUID, name, website, version, image_list')
+#DatData = collections.namedtuple('DatData', 'UUID, name, website, version, image_list')
+
+class MetadataDBSessionException(Exception):
+    '''This exception is raised when something goes wrong with a database session.'''
 
 
 def _uuidgen():
@@ -29,7 +31,6 @@ _SQLBase = sqlalchemy.ext.declarative.declarative_base()
 
 class Release(_SQLBase):
     '''SQLAlchemy ORM class for ROM image metadata.'''
-    # TODO: Split filenames into more meaningful metadata.
     __tablename__ = 'images'
     id = sqlalchemy.Column(sqlalchemy.Integer, sqlalchemy.Sequence('image_id_sequence'),
                            primary_key=True)
@@ -94,6 +95,7 @@ def configure(db_path):
 
     _SQLBase.metadata.create_all(_engine)
     _db_session_maker.configure(bind=_engine)
+    _configured = True
 
 # TODO: Passing the session object is a little clunky. Maybe there's a way to infer it somehow?
 def search(session, table_object, **constraints):
@@ -120,7 +122,10 @@ def search(session, table_object, **constraints):
 @contextlib.contextmanager
 def get_db_session():
     '''Get a SQLAlchemy database session with a proper context object. '''
-    # TODO: Raise an exception if _db_session_maker() isn't configured.
+    # TODO: There's probably a more reliable way of knowing whether the database was configured.
+    if not _configured:
+        raise MetadataDBSessionException('Tried to get session without configuring a database.')
+
     session = _db_session_maker()
     try:
         yield session