Add staged_files_only context manager.

pre_commit/prefixed_command_runner.py

@@ -6,6 +6,9 @@ import subprocess
 
 class CalledProcessError(RuntimeError):
     def __init__(self, returncode, cmd, expected_returncode, output=None):
+        super(CalledProcessError, self).__init__(
+            returncode, cmd, expected_returncode, output,
+        )
         self.returncode = returncode
         self.cmd = cmd
         self.expected_returncode = expected_returncode
@@ -15,13 +18,13 @@ class CalledProcessError(RuntimeError):
         return (
             'Command: {0!r}\n'
             'Return code: {1}\n'
-            'Expected return code {2}\n',
+            'Expected return code: {2}\n'
             'Output: {3!r}\n'.format(
                 self.cmd,
                 self.returncode,
                 self.expected_returncode,
                 self.output,
-            ),
+            )
         )
 
 
@@ -48,15 +51,15 @@ class PrefixedCommandRunner(object):
             self.__makedirs(self.prefix_dir)
 
     def run(self, cmd, retcode=0, stdin=None, **kwargs):
+        popen_kwargs = {
+            'stdin': subprocess.PIPE,
+            'stdout': subprocess.PIPE,
+            'stderr': subprocess.PIPE,
+        }
+        popen_kwargs.update(kwargs)
         self._create_path_if_not_exists()
         replaced_cmd = _replace_cmd(cmd, prefix=self.prefix_dir)
-        proc = self.__popen(
-            replaced_cmd,
-            stdin=subprocess.PIPE,
-            stdout=subprocess.PIPE,
-            stderr=subprocess.PIPE,
-            **kwargs
-        )
+        proc = self.__popen(replaced_cmd, **popen_kwargs)
         stdout, stderr = proc.communicate(stdin)
         returncode = proc.returncode