[Android] Prepare build/android/ for catapult+devil.

build/android/pylib/constants/host_paths.py

+# Copyright 2016 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.
+
+import contextlib
+import os
+import sys
+
+DIR_SOURCE_ROOT = os.environ.get(
+    'CHECKOUT_SOURCE_ROOT',
+    os.path.abspath(os.path.join(os.path.dirname(__file__),
+                                 os.pardir, os.pardir, os.pardir, os.pardir)))
+
+BUILD_COMMON_PATH = os.path.join(
+    DIR_SOURCE_ROOT, 'build', 'util', 'lib', 'common')
+
+# third-party libraries
+ANDROID_PLATFORM_DEVELOPMENT_SCRIPTS_PATH = os.path.join(
+    DIR_SOURCE_ROOT, 'third_party', 'android_platform', 'development',
+    'scripts')
+DEVIL_PATH = os.path.join(
+    DIR_SOURCE_ROOT, 'third_party', 'catapult', 'devil')
+PYMOCK_PATH = os.path.join(
+    DIR_SOURCE_ROOT, 'third_party', 'pymock')
+
+@contextlib.contextmanager
+def SysPath(path, position=None):
+  if position is None:

[agrieve, 2016/01/08 21:04:03]

nit: might be better to make inserting at the beginning the default so that you
always import the thing you just explicitly asked for.

[jbudorick, 2016/01/08 23:10:21]

I'd rather stick with appends and allow the few users that need to deviate to do
so. I really don't want to accidentally override a standard library import.

+    sys.path.append(path)
+  else:
+    sys.path.insert(position, path)
+  try:
+    yield
+  finally:
+    if sys.path[-1] == path:
+      sys.path.pop()
+    else:
+      sys.path.remove(path)