diff --git a/pre_commit_hooks/util.py b/pre_commit_hooks/util.py
index 269b553..5d1d11b 100644
--- a/pre_commit_hooks/util.py
+++ b/pre_commit_hooks/util.py
@@ -3,23 +3,25 @@
 from __future__ import unicode_literals
 
 import subprocess
+from typing import Any
+from typing import Set
 
 
 class CalledProcessError(RuntimeError):
     pass
 
 
-def added_files():
+def added_files():  # type: () -> Set[str]
     return set(cmd_output(
         'git', 'diff', '--staged', '--name-only', '--diff-filter=A',
     ).splitlines())
 
 
-def cmd_output(*cmd, **kwargs):
+def cmd_output(*cmd, **kwargs):  # type: (*str, **Any) -> str
     retcode = kwargs.pop('retcode', 0)
-    popen_kwargs = {'stdout': subprocess.PIPE, 'stderr': subprocess.PIPE}
-    popen_kwargs.update(kwargs)
-    proc = subprocess.Popen(cmd, **popen_kwargs)
+    kwargs.setdefault('stdout', subprocess.PIPE)
+    kwargs.setdefault('stderr', subprocess.PIPE)
+    proc = subprocess.Popen(cmd, **kwargs)
     stdout, stderr = proc.communicate()
     stdout = stdout.decode('UTF-8')
     if stderr is not None: