diff -uNrp a/Lib/test/test_urlparse.py b/Lib/test/test_urlparse.py
--- a/Lib/test/test_urlparse.py	2019-12-21 15:54:46.576000000 +0800
+++ b/Lib/test/test_urlparse.py	2019-12-21 15:56:22.440000000 +0800
@@ -648,11 +648,13 @@ class UrlParseTestCase(unittest.TestCase
             urlparse.urlsplit(u'http://\u30d5\u309a\ufe1380')
 
         for scheme in [u"http", u"https", u"ftp"]:
-            for c in denorm_chars:
-                url = u"{}://netloc{}false.netloc/path".format(scheme, c)
-                print "Checking %r" % url
-                with self.assertRaises(ValueError):
-                    urlparse.urlsplit(url)
+            for netloc in [u"netloc{}false.netloc", u"n{}user@netloc"]:
+                for c in denorm_chars:
+                    url = u"{}://{}/path".format(scheme, netloc.format(c))
+                    if test_support.verbose:
+                        print "Checking %r" % url
+                    with self.assertRaises(ValueError):
+                        urlparse.urlsplit(url)
 
 def test_main():
     test_support.run_unittest(UrlParseTestCase)
diff -uNrp a/Lib/urlparse.py b/Lib/urlparse.py
--- a/Lib/urlparse.py	2019-12-21 15:54:46.344000000 +0800
+++ b/Lib/urlparse.py	2019-12-21 15:57:41.260000000 +0800
@@ -171,18 +171,18 @@ def _checknetloc(netloc):
     # looking for characters like \u2100 that expand to 'a/c'
     # IDNA uses NFKC equivalence, so normalize for this check
     import unicodedata
-    n = netloc.rpartition('@')[2] # ignore anything to the left of '@'
-    n = n.replace(':', '')        # ignore characters already included
-    n = n.replace('#', '')        # but not the surrounding text
-    n = n.replace('?', '')
+    n = netloc.replace(u'@', u'') # ignore characters already included
+    n = n.replace(u':', u'')      # but not the surrounding text
+    n = n.replace(u'#', u'')
+    n = n.replace(u'?', u'')
     netloc2 = unicodedata.normalize('NFKC', n)
     if n == netloc2:
         return
     _, _, netloc = netloc.rpartition('@') # anything to the left of '@' is okay
     for c in '/?#@:':
         if c in netloc2:
-            raise ValueError("netloc '" + netloc + "' contains invalid " +
-                             "characters under NFKC normalization")
+            raise ValueError(u"netloc '" + netloc + u"' contains invalid " +
+                             u"characters under NFKC normalization")
 
 def urlsplit(url, scheme='', allow_fragments=True):
     """Parse a URL into 5 components: