diff --git a/gerrit_client.py b/gerrit_client.py
index ccf78b56a..6b3e9dee3 100755
--- a/gerrit_client.py
+++ b/gerrit_client.py
@@ -16,8 +16,13 @@ import logging
 import optparse
 import subcommand
 import sys
-import urllib
-import urlparse
+
+if sys.version_info.major == 2:
+  import urlparse
+  from urllib import quote_plus
+else:
+  from urllib.parse import quote_plus
+  import urllib.parse as urlparse
 
 import fix_encoding
 import gerrit_util
@@ -38,8 +43,8 @@ def CMDbranchinfo(parser, args):
 
   (opt, args) = parser.parse_args(args)
   host = urlparse.urlparse(opt.host).netloc
-  project = urllib.quote_plus(opt.project)
-  branch = urllib.quote_plus(opt.branch)
+  project = quote_plus(opt.project)
+  branch = quote_plus(opt.branch)
   result = gerrit_util.GetGerritBranch(host, project, branch)
   logging.info(result)
   write_result(result, opt)
@@ -51,11 +56,14 @@ def CMDbranch(parser, args):
   parser.add_option('--commit', dest='commit', help='commit hash')
 
   (opt, args) = parser.parse_args(args)
+  assert opt.project, "--project not defined"
+  assert opt.branch, "--branch not defined"
+  assert opt.commit, "--commit not defined"
 
-  project = urllib.quote_plus(opt.project)
+  project = quote_plus(opt.project)
   host = urlparse.urlparse(opt.host).netloc
-  branch = urllib.quote_plus(opt.branch)
-  commit = urllib.quote_plus(opt.commit)
+  branch = quote_plus(opt.branch)
+  commit = quote_plus(opt.commit)
   result = gerrit_util.CreateGerritBranch(host, project, branch, commit)
   logging.info(result)
   write_result(result, opt)
@@ -92,6 +100,7 @@ def CMDabandon(parser, args):
   parser.add_option('-m', '--message', default='', help='reason for abandoning')
 
   (opt, args) = parser.parse_args(args)
+  assert opt.change, "-c not defined"
   result = gerrit_util.AbandonChange(
       urlparse.urlparse(opt.host).netloc,
       opt.change, opt.message)
@@ -102,8 +111,7 @@ def CMDabandon(parser, args):
 class OptionParser(optparse.OptionParser):
   """Creates the option parse and add --verbose support."""
   def __init__(self, *args, **kwargs):
-    optparse.OptionParser.__init__(
-        self, *args, prog='git cl', version=__version__, **kwargs)
+    optparse.OptionParser.__init__(self, *args, version=__version__, **kwargs)
     self.add_option(
         '--verbose', action='count', default=0,
         help='Use 2 times for more debugging info')
@@ -114,6 +122,8 @@ class OptionParser(optparse.OptionParser):
 
   def parse_args(self, args=None, values=None):
     options, args = optparse.OptionParser.parse_args(self, args, values)
+    # Host is always required
+    assert options.host, "--host not defined."
     levels = [logging.WARNING, logging.INFO, logging.DEBUG]
     logging.basicConfig(level=levels[min(options.verbose, len(levels) - 1)])
     return options, args
diff --git a/tests/gerrit_client_test.py b/tests/gerrit_client_test.py
new file mode 100755
index 000000000..90b0a8963
--- /dev/null
+++ b/tests/gerrit_client_test.py
@@ -0,0 +1,68 @@
+#!/usr/bin/env vpython3
+# coding=utf-8
+# Copyright 2020 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+"""Unit tests for gerrit_client.py."""
+
+import logging
+import os
+import sys
+import unittest
+
+if sys.version_info.major == 2:
+  from StringIO import StringIO
+  import mock
+else:
+  from io import StringIO
+  from unittest import mock
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+import gerrit_client
+import gerrit_util
+
+
+class TestGerritClient(unittest.TestCase):
+  @mock.patch('gerrit_util.GetGerritBranch', return_value='')
+  def test_branch_info(self, util_mock):
+    gerrit_client.main([
+        'branchinfo', '--host', 'https://example.org/foo', '--project',
+        'projectname', '--branch', 'branchname'
+    ])
+    util_mock.assert_called_once_with('example.org', 'projectname',
+                                      'branchname')
+
+  @mock.patch('gerrit_util.CreateGerritBranch', return_value='')
+  def test_branch(self, util_mock):
+    gerrit_client.main([
+        'branch', '--host', 'https://example.org/foo', '--project',
+        'projectname', '--branch', 'branchname', '--commit', 'commitname'
+    ])
+    util_mock.assert_called_once_with('example.org', 'projectname',
+                                      'branchname', 'commitname')
+
+  @mock.patch('gerrit_util.QueryChanges', return_value='')
+  def test_changes(self, util_mock):
+    gerrit_client.main([
+        'changes', '--host', 'https://example.org/foo', '-p', 'foo=bar', '-p',
+        'baz=qux', '--limit', '10', '--start', '20', '-o', 'op1', '-o', 'op2'
+    ])
+    util_mock.assert_called_once_with(
+        'example.org', [('foo', 'bar'), ('baz', 'qux')],
+        limit=10,
+        start=20,
+        o_params=['op1', 'op2'])
+
+  @mock.patch('gerrit_util.AbandonChange', return_value='')
+  def test_abandon(self, util_mock):
+    gerrit_client.main([
+        'abandon', '--host', 'https://example.org/foo', '-c', '1', '-m', 'bar'
+    ])
+    util_mock.assert_called_once_with('example.org', 1, 'bar')
+
+
+if __name__ == '__main__':
+  logging.basicConfig(
+      level=logging.DEBUG if '-v' in sys.argv else logging.ERROR)
+  unittest.main()