Adds db layer.

src/couchpotato/db.py

@@ -0,0 +1,64 @@
+import sqlalchemy as sa
+from sqlalchemy import orm
+
+
+class DatabaseError(Exception):
+    """Custom exceptions related to the database."""
+    pass
+
+
+def _session_cls_cache(cache={}):
+    """Holds a dictionary to cache session objects."""
+    return cache
+
+
+def get_session(engine=None, test=False):
+    """
+    Get the current session or create a new one based on the engine.
+
+    >>> from couchpotato import db
+    >>> from sqlalchemy import create_engine
+    >>> engine = create_engine('sqlite:///:memory:')
+    >>> session = db.get_session(engine)
+    >>> session #doctest: +ELLIPSIS
+    <sqlalchemy.orm.session.Session object at ...>
+
+    Once a session has been created, get_session will return session instances
+    of the same Session class.
+    >>> type(session) == type(db.get_session())
+    True
+
+    If you create multiple sessions for different engines, you need to
+    specify which session you want by passing the engine explicitely.
+
+    >>> other_engine = create_engine('sqlite:///:memory:')
+    >>> other_session = db.get_session(other_engine)
+    >>> type(other_session) is type(db.get_session(other_engine))
+    True
+
+    """
+    cache = _session_cls_cache()
+
+    assert not(engine and test), "Cannot pass both test and engine."
+    # It doesn't make sense to both pass an engine and instruct the function
+    # to create a new engine.  Decide what you want to do, but not both.
+    if test:
+        in_memory = sa.create_engine('sqlite:///:memory:')
+        session = orm.sessionmaker(bind=in_memory)()
+        # create Session class ^               ^
+        #              create Session instance ^
+    elif engine:
+        key = (engine, )
+        if key not in cache:
+            cache[key] = orm.sessionmaker(bind=engine)
+        session = cache[key]()
+    elif len(cache) == 1:
+        session = (cache[key] for key in cache).next()()
+        #             return the first element ^     ^
+        #                        instantiate session ^
+    elif len(cache) >= 1:
+        raise DatabaseError("Multiple Session classes found. Choose one.")
+    else:
+        raise DatabaseError("No session found. You need to create one.")
+
+    return session