add very basic singleton class

Signed-off-by: Mark Hymers <mhy@debian.org>

diff --git a/daklib/Singleton.py b/daklib/Singleton.py
new file mode 100644 (file)
index 0000000..456d2cc
--- /dev/null
+++ b/daklib/Singleton.py
@@ -0,0 +1,63 @@
+#!/usr/bin/env python
+# vim:set et ts=4 sw=4:
+
+# Singleton pattern code
+# Copyright (C) 2008  Mark Hymers <mhy@debian.org>
+
+# Inspiration for this very simple ABC was taken from various documents /
+# tutorials / mailing lists.  This may not be thread safe but given that
+# (as I write) large chunks of dak aren't even type-safe, I'll live with
+# it for now
+
+################################################################################
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+################################################################################
+
+# < sgran> NCommander: in SQL, it's better to join than to repeat information
+# < tomv_w> that makes SQL the opposite to Debian mailing lists!
+
+################################################################################
+
+"""
+This class set implements objects that may need to be instantiated multiple
+times, but we don't want the overhead of actually creating and init'ing
+them more than once.  It also saves us using globals all over the place
+"""
+
+class Singleton(object):
+    """This is the ABC for other dak Singleton classes"""
+    __single = None
+    def __new__(cls, *args, **kwargs):
+        # Check to see if a __single exists already for this class
+        # Compare class types instead of just looking for None so
+        # that subclasses will create their own __single objects
+        if cls != type(cls.__single):
+            cls.__single = object.__new__(cls, *args, **kwargs)
+            cls.__single._startup(*args, **kwargs)
+        return cls.__single
+
+    def __init__(self, *args, **kwargs):
+        if type(self) == "Singleton":
+            raise NotImplementedError("Singleton is an ABC")
+
+    def _startup(self):
+        """
+        _startup is a private method used instead of __init__ due to the way
+        we instantiate this object
+        """
+        raise NotImplementedError("Singleton is an ABC")
+