Create global instance of command-line options

This eases testing of option values inside the code.  This instance
is implemented as the read-only copy of the obtained 'options' object,
so callers won't be able to modify its contents.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/utils/const.py

@@ -0,0 +1,40 @@
+# Copyright 2013 Eygene A. Ryabinkin.
+#
+# Collection of classes that implement const-like behaviour
+# for various objects.
+
+import copy
+
+class ConstProxy (object):
+	"""
+	Implements read-only access to a given object
+	that can be attached to each instance only once.
+
+	"""
+
+	def __init__ (self):
+		self.__dict__['__source'] = None
+
+
+	def __getattr__ (self, name):
+		src = self.__dict__['__source']
+		if src == None:
+			raise ValueError ("using non-initialized ConstProxy() object")
+		return copy.deepcopy (getattr (src, name))
+
+
+	def __setattr__ (self, name, value):
+		raise AttributeError ("tried to set '%s' to '%s' for constant object" % \
+		  (name, value))
+
+
+	def __delattr__ (self, name):
+		raise RuntimeError ("tried to delete field '%s' from constant object" % \
+		  (name))
+
+
+	def set_source (self, source):
+		""" Sets source object for this instance. """
+		if (self.__dict__['__source'] != None):
+			raise ValueError ("source object is already set")
+		self.__dict__['__source'] = source