Say you are comparing two large dicts in a unit test, for example:

    form = extract_form(rendered_html)
    self.assertEquals(form, {'field1': u'value1',
                             'field2': u'value2',
                             'field42': u'value42'})

When this test fails, a useful trick is to ask the test runner to drop into Pdb inside assertEquals (SchoolTool and Zope 3 test runners have a command line option -d for this) and type the following:

(Pdb) from sets import Set
(Pdb) pp list(Set(first.items()) ^ Set(second.items()))

You will get a list of (key, value) pairs that differ:

[('field.comp.c3.b.NEW', u''),
 ('field.comp.c1.b.NEW', u''),
 ('field.comp.c1.title', 'New stuff'),
 ('field.comp.c1.b.b2', u'A2'),
 ('field.comp.c3.b.b1', u'New behaviour'),
 ('field.comp.c3.title', u'New stuff'),
 ('SUBMIT', u'Save'),
 ('field.comp.c1.title', u'Comp 1'),
 ('field.comp.c2.b.b1', 'B1'),
 ('field.comp.c3.description', u'New description'),
 ('field.comp.c2.description', 'Comp two'),
 ('field.comp.c1.b.b1', u'A1'),
 ('field.comp.c2.title', 'Comp 2'),
 ('SUBMIT', 'Submit'),
 ('field.comp.c2.b.b2', 'B2'),
 ('field.comp.c1.description', 'New description'),
 ('field.comp.c1.description', u'Comp one'),
 ('field.comp.c1.b.b1', 'New behaviour')]

If there are many differences, sorting the list is a good idea

(Pdb) sorted = lambda l: (l.sort(), l)[1]
(Pdb) pp sorted(list(Set(first.items()) ^ Set(second.items())))

Python 2.4 makes this simpler (builtin set, builtin sorted).