diff --git a/ptvsd/debugger.py b/ptvsd/debugger.py
index 399d711c..7ac7871d 100644
--- a/ptvsd/debugger.py
+++ b/ptvsd/debugger.py
@@ -4,6 +4,10 @@
 
 import sys
 
+# import the wrapper first, so that it gets a chance
+# to detour pydevd socket functionality.
+import ptvsd.wrapper
+
 
 __author__ = "Microsoft Corporation <ptvshelp@microsoft.com>"
 __version__ = "4.0.0a1"
@@ -13,22 +17,39 @@ DONT_DEBUG = []
 
 def debug(filename, port_num, debug_id, debug_options, run_as):
     # TODO: docstring
-
-    # import the wrapper first, so that it gets a chance
-    # to detour pydevd socket functionality.
-    import ptvsd.wrapper
-    import pydevd
-
-    args = [
-            '--port', str(port_num),
-            '--client', '127.0.0.1',
-            ]
+    address = (None, port_num)
     if run_as == 'module':
-        args.append('--module')
-        args.extend(('--file', filename + ":"))
+        _run_module(address, filename)
     else:
-        args.extend(('--file', filename))
-    sys.argv[1:0] = args
+        _run_file(address, filename)
+
+
+def _run_module(address, modname):
+    filename = modname + ':'
+    argv = _run_argv(address, filename)
+    argv.append('--module')
+    _run(argv)
+
+
+def _run_file(address, filename):
+    argv = _run_argv(address, filename)
+    _run(argv)
+
+
+def _run_argv(address, filename):
+    host, port = address
+    if host is None:
+        host = '127.0.0.1'
+    return [
+        '--port', str(port),
+        '--client', host,
+        '--file', filename,
+    ]
+
+
+def _run(argv):
+    import pydevd
+    sys.argv[1:0] = argv
     try:
         pydevd.main()
     except SystemExit as ex: