--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/python/optparse_undefined.py	Fri Jan 20 14:15:16 2012 -0800
@@ -0,0 +1,49 @@
+#!/usr/bin/env python
+
+"""
+illustrates usage of the undefined pattern from e.g.
+http://k0s.org/mozilla/hg/bzconsole/file/tip/bzconsole/command.py
+
+This is useful for scenarios similar to:
+- you have base configuration in a file
+- you have an OptionParser to read options in the command line
+- the CLI options should overwrite the file configuration iff
+  the user specifies an option
+"""
+
+from optparse import OptionParser
+
+class Undefined(object):
+    def __init__(self, default):
+        self.default=default
+
+class UndefinedOptionParser(OptionParser):
+
+    def add_option(self, *args, **kwargs):
+        kwargs['default'] = Undefined(kwargs.get('default'))
+        OptionParser.add_option(self, *args, **kwargs)
+
+    def parse_args(self, *args, **kwargs):
+        options, args = OptionParser.parse_args(self, *args, **kwargs)
+        return options, args
+
+if __name__ == '__main__':
+
+    myparser = UndefinedOptionParser()
+    myparser.add_option("--foo", dest='foo', default='hello')
+    myparser.add_option("--bar", dest='bar', default='goodbye')
+    myparser.add_option("--baz", dest='baz', default='helloagain')
+    myparser.add_option("--fleem", dest='fleem', default='aufwiedersehen')
+
+    myconfiguration = {'foo': 'hi',
+                       'bar': 'ciao'}
+    options, args = myparser.parse_args(['--foo', 'hello', '--baz', 'hola'])
+    for key, value in options.__dict__.items():
+        # XXX ideally you would move this to parse_args
+        if isinstance(value, Undefined):
+            options.__dict__[key] = myconfiguration.get(key, value.default)
+
+    assert options.foo == 'hello'
+    assert options.bar == 'ciao'
+    assert options.baz == 'hola'
+    assert options.fleem == 'aufwiedersehen'