Allow base path overrides when running a recipe.

recipe_modules/path/api.py

 # Copyright 2013 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 functools
 import os
 import sys
 import tempfile
 
 from recipe_engine import recipe_api
@@ skipping 107 matching lines @@
 def _split_path(path):  # pragma: no cover
   """Relative or absolute path -> tuple of components."""
   abs_path = os.path.abspath(path).split(os.path.sep)
   # Guarantee that the first element is an absolute drive or the posix root.
   if abs_path[0].endswith(':'):
     abs_path[0] += '\\'
   elif abs_path[0] == '':
     abs_path[0] = '/'
   else:
     assert False, 'Got unexpected path format: %r' % abs_path
-  return abs_path
+  return tuple(abs_path)
 
 
 class PathApi(recipe_api.RecipeApi):
   """
   PathApi provides common os.path functions as well as convenience functions
   for generating absolute paths to things in a testable way.
 
   Mocks:
     exists (list): Paths which should exist in the test case. Thes must be paths
       using the [*_ROOT] placeholders. ex. '[BUILD_ROOT]/scripts'.
   """
 
   OK_ATTRS = ('pardir', 'sep', 'pathsep')
 
   # Because the native 'path' type in python is a str, we filter the *args
   # of these methods to stringify them first (otherwise they would be getting
   # recipe_util_types.Path instances).
   FILTER_METHODS = ('abspath', 'basename', 'exists', 'join', 'split',
                     'splitext')
 
   def get_config_defaults(self):
     return {
       'CURRENT_WORKING_DIR': self._startup_cwd,
       'TEMP_DIR': self._temp_dir,
+      'BASE_PATH_OVERRIDES': {
+        k: _split_path(v)
+        for k, v in
+            self._engine.properties.get('base_path_overrides', {}).iteritems()
+      },
     }
 
   def __init__(self, **kwargs):
     super(PathApi, self).__init__(**kwargs)
     config_types.Path.set_tostring_fn(
       PathToString(self, self._test_data))
 
     # Used in mkdtemp when generating and checking expectations.
     self._test_counter = 0
 
@@ skipping 67 matching lines @@
     if name in self.OK_ATTRS:
       return getattr(self._path_mod, name)
     if name in self.FILTER_METHODS:
       return string_filter(getattr(self._path_mod, name))
     raise AttributeError("'%s' object has no attribute '%s'" %
                          (self._path_mod, name))  # pragma: no cover
 
   def __dir__(self):  # pragma: no cover
     # Used for helping out show_me_the_modules.py
     return self.__dict__.keys() + list(self.OK_ATTRS + self.FILTER_METHODS)