add snowflake as sqlalchemy duct protocol (#64)

omniduct/_version.py

@@ -52,6 +52,10 @@
         'pyspark',  # Primary client
     ],
 
+    'snowflake': [
+        'snowflake-sqlalchemy',
+    ],
+
     # Filesystems
     'webhdfs': [
         'pywebhdfs',  # Primary client

omniduct/databases/sqlalchemy.py

@@ -8,12 +8,12 @@
 
 class SQLAlchemyClient(DatabaseClient, SchemasMixin):
 
-    PROTOCOLS = ['sqlalchemy', 'firebird', 'mssql', 'mysql', 'oracle', 'postgresql', 'sybase']
+    PROTOCOLS = ['sqlalchemy', 'firebird', 'mssql', 'mysql', 'oracle', 'postgresql', 'sybase', 'snowflake']
     NAMESPACE_NAMES = ['database', 'table']
     NAMESPACE_QUOTECHAR = '"'  # TODO: Apply overrides depending on protocol?
     NAMESPACE_SEPARATOR = '.'
 
-    def _init(self, dialect=None, driver=None, database=''):
+    def _init(self, dialect=None, driver=None, database='', engine_opts=None):
 
         assert self._port is not None, "Omniduct requires SQLAlchemy databases to manually specify a port, as " \
                                        "it will often be the case that ports are being forwarded."
@@ -27,6 +27,7 @@ def _init(self, dialect=None, driver=None, database=''):
         self.driver = driver
         self.database = database
         self.connection_fields += ('schema',)
+        self.engine_opts = engine_opts or {}
 
         self.engine = None
         self.connection = None
@@ -43,7 +44,7 @@ def db_uri(self):
     def _connect(self):
         import sqlalchemy
 
-        self.engine = sqlalchemy.create_engine(self.db_uri)
+        self.engine = sqlalchemy.create_engine(self.db_uri, **self.engine_opts)
         self._sqlalchemy_metadata = sqlalchemy.MetaData(self.engine)
 
     def _is_connected(self):

omniduct/duct.py

@@ -237,13 +237,13 @@ class Duct(with_metaclass(ProtocolRegisteringQuirkDocumentedABCMeta, object)):
     class Type(Enum):
         """
         The `Duct.Type` enum specifies all of the permissible values of
-        `Duct.DUCT_TYPE`.
+        `Duct.DUCT_TYPE`. Also determines the order in which ducts are loaded by DuctRegistry.
         """
         REMOTE = 'remotes'
         FILESYSTEM = 'filesystems'
-        DATABASE = 'databases'
         CACHE = 'caches'
         RESTFUL = 'rest_clients'
+        DATABASE = 'databases'
         OTHER = 'other'
 
     AUTO_LOGGING_SCOPE = True
@@ -405,6 +405,7 @@ def _prepare(self):
            and '_password'.
          - Ensures value of self.port is an integer (or None).
         """
+
         # Import necessary classes lazily (to prevent dependency cycles)
         from omniduct.registry import DuctRegistry
         from omniduct.caches.base import Cache