import urllib
try:
    from urlparse import parse_qs
except ImportError:
    from cgi import parse_qs

def url_params(url, **kwargs):
    bits = url.split('?')
    query_vars = {}
    if len(bits) > 1:
        query_vars = parse_qs(bits[1], keep_blank_values=1)
    for key, value in query_vars.items():
        # value is always a list - kill it as we want to overwrite it
        query_vars[key] = value[0]
    query_vars.update(kwargs)
    new_dict = {}
    for key, value in query_vars.items():
        # filter out empty values
        if str(value) != '0' and not value:
            continue
        else:
            new_dict[key] = value
    kwargs = new_dict
    query_string = ''
    url = bits[0]
    if kwargs:
        query_string = urllib.urlencode(kwargs)
        if query_string:
            url = '%s?%s' % (url, query_string)
    return url
    
if __name__ == '__main__':
    import unittest
    class UrlTest(unittest.TestCase):
        def test_single_query_string(self):
            """Tests that a query string is appended correctly"""
            new_url = url_params('/something/something', blah='true')
            self.assertEqual(new_url, '/something/something?blah=true')

        def test_multi_query_string(self):
            """Tests that multiple parameters to a query string are appended"""
            new_url = url_params('/something/something', blah='true', something='somethingelse')
            self.assertEqual(new_url, '/something/something?blah=true&something=somethingelse')

        def test_no_query_string(self):
            """Tests that no query string is valid"""
            new_url = url_params('/blah/foo')
            self.assertEqual(new_url, '/blah/foo')

        def test_existing_query_string(self):
            """Tests that old query strings aren't overwritten"""
            new_url = url_params('/blah/foo?thing=other&blah=yes')
            self.assertEqual(new_url, '/blah/foo?thing=other&blah=yes')

        def test_overwrite_query_string(self):
            """tests that query parameters are overwritten by arguments"""
            new_url = url_params('/blah/foo?thing=yes', thing='no')
            self.assertEqual(new_url, '/blah/foo?thing=no')

        def test_delete_query_string(self):
            """tests that query parameters are deleted by explicitly killing them"""
            new_url = url_params('/blah/foo?thing=yes', thing=False)
            self.assertEqual(new_url, '/blah/foo')

        def test_extra_parameter(self):
            """Tests that extra false parameters don't kill the reverser"""
            new_url = url_params('/blah/foo', thing=False)
            self.assertEqual(new_url, '/blah/foo')

        def test_falsiness(self):
            """Test that only False, empty strings, and None gets dropped"""
            new_url = url_params('/blah/foo', thing=None)
            self.assertEqual(new_url, '/blah/foo')
            new_url = url_params('/blah/foo', thing=False)
            self.assertEqual(new_url, '/blah/foo')
            new_url = url_params('/blah/foo', thing='')
            self.assertEqual(new_url, '/blah/foo')
            new_url = url_params('/blah/foo', thing=0)
            self.assertEqual(new_url, '/blah/foo?thing=0')
            new_url = url_params('/blah/foo', thing=1)
            self.assertEqual(new_url, '/blah/foo?thing=1')

    unittest.main()