Add a test for kernel started without user_ns kwarg.

user_module should not be an instance of DummyMod

IPython/kernel/zmq/tests/test_start_kernel.py

@@ -8,10 +8,38 b' def test_ipython_start_kernel_userns():'
     cmd = ('from IPython import start_kernel\n'
            'ns = {"tre": 123}\n'
            'start_kernel(user_ns=ns)')
-    
+
     with setup_kernel(cmd) as client:
         msg_id = client.object_info('tre')
         msg = client.get_shell_msg(block=True, timeout=TIMEOUT)
         content = msg['content']
         assert content['found']
-        nt.assert_equal(content['string_form'], u'123')
  No newline at end of file
+        nt.assert_equal(content['string_form'], u'123')
+
+        # user_module should be an instance of DummyMod
+        msg_id = client.execute("usermod = get_ipython().user_module")
+        msg = client.get_shell_msg(block=True, timeout=TIMEOUT)
+        content = msg['content']
+        nt.assert_equal(content['status'], u'ok')
+        msg_id = client.object_info('usermod')
+        msg = client.get_shell_msg(block=True, timeout=TIMEOUT)
+        content = msg['content']
+        assert content['found']
+        nt.assert_in('DummyMod', content['string_form'])
+
+def test_ipython_start_kernel_no_userns():
+    # Issue #4188 - user_ns should be passed to shell as None, not {}
+    cmd = ('from IPython import start_kernel\n'
+           'start_kernel()')
+
+    with setup_kernel(cmd) as client:
+        # user_module should not be an instance of DummyMod
+        msg_id = client.execute("usermod = get_ipython().user_module")
+        msg = client.get_shell_msg(block=True, timeout=TIMEOUT)
+        content = msg['content']
+        nt.assert_equal(content['status'], u'ok')
+        msg_id = client.object_info('usermod')
+        msg = client.get_shell_msg(block=True, timeout=TIMEOUT)
+        content = msg['content']
+        assert content['found']
+        nt.assert_not_in('DummyMod', content['string_form'])