More declarative handling of the database types

2 files changed, 56 insertions, 41 deletions

diff --git a/portato/db/__init__.py b/portato/db/__init__.py
index 21d8f80..bb5c6fc 100644
--- a/portato/db/__init__.py
+++ b/portato/db/__init__.py
@@ -12,66 +12,79 @@
 
 from __future__ import absolute_import
 
+from collections import namedtuple
+
 from . import database as db
-from .exceptions import UnknownDatabaseTypeError, DatabaseInstantiationError
+from .exceptions import UnknownDatabaseTypeError, DatabaseInstantiationError, DatabaseInitError
 from ..session import Session, SectionDict
 from ..helper import debug, warning, error, info
+from ..odict import OrderedDict
+
+DBType = namedtuple("DBType", "name descr module cls alt")
+
+types = [
+        ("eixsql",
+                DBType(
+                    _("eix + SQLite"),
+                    _("Similar to SQLite, but now uses the eix database to get the package information.\nThis should be much faster on startup, but requires that your eix database is always up-to-date.\nAdditionally, this is the only database allowing searching in descriptions."),
+                    "eix_sql", "EixSQLDatabase",
+                    "sql")),
+        ("sql",
+                DBType(
+                    _("SQLite"),
+                    _("Uses an SQLite-database to store package information.\nMay take longer to generate at the first time, but has advantages if portato is re-started with an unchanged portage tree. Additionally it allows to use fast SQL expressions for fetching the data."),
+                    "sql", "SQLDatabase",
+                    "dict")),
+        ("dict",
+                DBType(
+                    _("Hashmap"),
+                    _("Uses an in-memory hashmap to store package information.\nHas been used since at least version 0.3.3, but all information has to be regenerated on each startup."),
+                    "hash", "HashDatabase",
+                    None))
+        ]
 
-types = (
-        ("eixsql", _("eix + SQLite"), _("Similar to SQLite, but now uses the eix database to get the package information.\nThis should be much faster on startup, but requires that your eix database is always up-to-date.\nAdditionally, this is the only database allowing searching in descriptions.")),
-        ("sql", _("SQLite"), _("Uses an SQLite-database to store package information.\nMay take longer to generate at the first time, but has advantages if portato is re-started with an unchanged portage tree. Additionally it allows to use fast SQL expressions for fetching the data.")),
-        ("dict", _("Hashmap"), _("Uses an in-memory hashmap to store package information.\nHas been used since at least version 0.3.3, but all information has to be regenerated on each startup."))
-        )
+types = OrderedDict(types)
 
 class Database(db.Database):
     DEFAULT = "dict"
 
     def __new__ (cls, type = None):
-        if not '_the_instance' in cls.__dict__:
-            dbcls = cls._generate(type)
-            cls._the_instance = dbcls(cls._get_session())
-        elif type is not None:
-            raise DatabaseInstantiationError("Database instantiation called with 'type' argument multiple times.")
+        try:
+            if not '_the_instance' in cls.__dict__:
+                dbcls = cls._generate(type)
+                cls._the_instance = dbcls(cls._get_session())
+            elif type is not None:
+                raise DatabaseInstantiationError("Database instantiation called with 'type' argument multiple times.")
+        except (ImportError, DatabaseInitError) as e:
+            db = types[cls.DB_TYPE]
+            error(_("Cannot load %s."), db.cls)
+            error(_("Error: %s"), e)
+
+            if db.alt is not None:
+                return cls.__new__(cls, db.alt)
+            else:
+                error(_("No alternative database given. Aborting."))
+                raise DatabaseInstantiationError("Cannot load database.")
+
         return cls._the_instance
     
     @classmethod
     def _generate(cls, type):
-
         if type is None:
             warning("No database type specified! Falling back to default.")
             type = cls.DEFAULT
-        
-        cls.DB_TYPE = type
-        msg = _("Using database type '%s'")
 
-        if type == "sql":
-            info(msg, "SQLDatabase")
-            try:
-                from .sql import SQLDatabase
-            except ImportError:
-                warning(_("Cannot load %s."), "SQLDatabase")
-                return cls._generate("dict")
-            else:
-                return SQLDatabase
-
-        elif type == "dict":
-            info(msg, "HashDatabase")
-            from .hash import HashDatabase
-            return HashDatabase
-        
-        elif type == "eixsql":
-            info(msg,"EixSQLDatabase")
-            try:
-                from .eix_sql import EixSQLDatabase
-            except ImportError:
-                warning(_("Cannot load %s."), "EixSQLDatabase.")
-                return cls._generate("sql")
-            else:
-                return EixSQLDatabase
-
-        else:
+        try:
+            db = types[type]
+        except KeyError:
             error(_("Unknown database type: %s"), type)
             raise UnknownDatabaseTypeError, type
+        
+        cls.DB_TYPE = type
+        info(_("Using database type '%s'"), db.cls)
+
+        mod = __import__(db.module, globals(), locals(), [db.cls])
+        return getattr(mod, db.cls)
 
     @classmethod
     def _get_session(cls):
diff --git a/portato/db/exceptions.py b/portato/db/exceptions.py
index 8a6e424..82fa337 100644
--- a/portato/db/exceptions.py
+++ b/portato/db/exceptions.py
@@ -22,3 +22,5 @@ class DatabaseInstantiationError (DatabaseError):
 class UnsupportedSearchTypeError(DatabaseError):
     pass
 
+class DatabaseInitError (DatabaseError):
+    pass