Add RunPylint as a canned presubmit check.

PRESUBMIT.py

 # Copyright (c) 2009 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.
 
 """Top-level presubmit script for depot tools.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for
 details on the presubmit API built into gcl.
 """
 
 UNIT_TESTS = [
   'tests.gcl_unittest',
   'tests.gclient_scm_test',
   'tests.gclient_smoketest',
   'tests.gclient_utils_test',
   'tests.presubmit_unittest',
   'tests.scm_unittest',
   'tests.trychange_unittest',
   'tests.watchlists_unittest',
 ]
 
+def CommonChecks(input_api, output_api):
+  output = []
+  output.extend(input_api.canned_checks.RunPythonUnitTests(
+      input_api,
+      output_api,
+      UNIT_TESTS))
+  output.extend(WasGitClUploadHookModified(input_api, output_api))
+
+  def filter_python_sources(affected_file):
+    filepath = affected_file.LocalPath()
+    return ((filepath.endswith('.py') and
+             filepath != 'cpplint.py' and
+             not filepath.startswith('tests')) or
+            filepath == 'git-try')
+
+  output.extend(input_api.canned_checks.RunPylint(
+      input_api,
+      output_api,
+      source_file_filter=filter_python_sources))
+  return output
+
+
 def CheckChangeOnUpload(input_api, output_api):
-  output = []
-  output.extend(input_api.canned_checks.RunPythonUnitTests(input_api,
-                                                           output_api,
-                                                           UNIT_TESTS))
-  output.extend(WasGitClUploadHookModified(input_api, output_api))
-  output.extend(RunPylint(input_api, output_api))
-  return output
+  return CommonChecks(input_api, output_api)
 
 
 def CheckChangeOnCommit(input_api, output_api):
   output = []
-  output.extend(input_api.canned_checks.RunPythonUnitTests(input_api,
-                                                           output_api,
-                                                           UNIT_TESTS))
-  output.extend(input_api.canned_checks.CheckDoNotSubmit(input_api,
-                                                         output_api))
-  output.extend(WasGitClUploadHookModified(input_api, output_api))
-  output.extend(RunPylint(input_api, output_api))
+  output.extend(CommonChecks(input_api, output_api))
+  output.extend(input_api.canned_checks.CheckDoNotSubmit(
+      input_api,
+      output_api))
   return output
 
+
 def WasGitClUploadHookModified(input_api, output_api):
   for affected_file in input_api.AffectedSourceFiles(None):
     if (input_api.os_path.basename(affected_file.LocalPath()) ==
         'git-cl-upload-hook'):
       return [output_api.PresubmitPromptWarning(
           'Don\'t forget to fix git-cl to download the newest version of '
           'git-cl-upload-hook')]
   return []
-
-def RunPylint(input_api, output_api):
-  import glob
-  files = glob.glob('*.py')
-  # It's a python script
-  files.append('git-try')
-  # It uses non-standard pylint exceptions that makes pylint always fail.
-  files.remove('cpplint.py')
-  try:
-    proc = input_api.subprocess.Popen(['pylint'] + sorted(files))
-    proc.communicate()
-    if proc.returncode:
-      return [output_api.PresubmitError('Fix pylint errors first.')]
-    return []
-  except OSError:
-    if input_api.platform == 'win32':
-      return [output_api.PresubmitNotifyResult(
-        'Warning: Can\'t run pylint because it is not installed. Please '
-        'install manually\n'
-        'Cannot do static analysis of python files.')]
-    return [output_api.PresubmitError(
-        'Please install pylint with "sudo apt-get install python-setuptools; '
-        'sudo easy_install pylint"\n'
-        'Cannot do static analysis of python files.')]