During presubmit, look in parent directories for buildtools/checkdeps.

PRESUBMIT.py

 # Copyright 2015 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.
 
 """Presubmit script for pdfium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into depot_tools.
 """
 
 def _CheckUnwantedDependencies(input_api, output_api):
   """Runs checkdeps on #include statements added in this
   change. Breaking - rules is an error, breaking ! rules is a
   warning.
   """
   import sys
   # We need to wait until we have an input_api object and use this
   # roundabout construct to import checkdeps because this file is
   # eval-ed and thus doesn't have __file__.
   original_sys_path = sys.path
   try:
-    sys.path = sys.path + [input_api.os_path.join(
-        input_api.PresubmitLocalPath(), 'buildtools', 'checkdeps')]
+    checkdeps_relpath = input_api.os_path.join('buildtools', 'checkdeps')
+    checkdeps_basepath = input_api.PresubmitLocalPath()
+    checkdeps_path = input_api.os_path.join(checkdeps_basepath,
+                                            checkdeps_relpath)
+    while not input_api.os_path.exists(checkdeps_path):
+      if checkdeps_basepath == input_api.os_path.dirname(checkdeps_basepath):

[Lei Zhang, 2015/11/20 02:57:42]

Well, in theory, /buildtools/checkdeps may contain the checkdeps we are looking
for, but really, that's not gonna happen.

+        raise ImportError('Cannot find checkdeps')
+      checkdeps_basepath = input_api.os_path.dirname(checkdeps_basepath)

[hans, 2015/11/20 03:26:28]

Sorry for being a little slow here.. how does this line work? Does it take
checkdeps_basepath to the parent directory?

[Lei Zhang, 2015/11/20 03:29:19]

It's essentially: foo = os.path.dirname(foo)
https://docs.python.org/2/library/os.path.html

[hans, 2015/11/20 03:52:21]

Ah, I get it now, thanks.

+      checkdeps_path = input_api.os_path.join(checkdeps_basepath,
+                                              checkdeps_relpath)
+
+    sys.path.append(checkdeps_path)
     import checkdeps
     from cpp_checker import CppChecker
     from rules import Rule
   finally:
     # Restore sys.path to what it was before.
     sys.path = original_sys_path
 
   added_includes = []
   for f in input_api.AffectedFiles():
     if not CppChecker.IsCppFile(f.LocalPath()):
@@ skipping 30 matching lines @@
 
 def CheckChangeOnUpload(input_api, output_api):
   results = []
   results += input_api.canned_checks.CheckPatchFormatted(input_api, output_api)
   results += _CheckUnwantedDependencies(input_api, output_api)
   return results
 
 
 def CheckChangeOnCommit(input_api, output_api):
   return CheckChangeOnUpload(input_api, output_api)