recipe engine: require all path bases to be classes

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 173 matching lines @@
   def __setitem__(self, pathname, path):
     assert isinstance(path, config_types.Path), (
       'Setting dynamic path to something other than a Path: %r' % path)
     assert pathname in self.c.dynamic_paths, (
       'Must declare dynamic path (%r) in config before setting it.' % path)
     assert isinstance(path.base, config_types.BasePath), (
       'Dynamic path values must be based on a base_path' % path.base)
     self.c.dynamic_paths[pathname] = path
 
   def __getitem__(self, name):
-    if name in self.c.dynamic_paths:
-      r = self.c.dynamic_paths[name]
-      assert r is not None, ('Tried to get dynamic path %s but it has not been '
-                             'set yet.' % name)
-      return r
-    if name in self.c.base_paths:
+    if name in self.c.base_paths or name in self.c.dynamic_paths:
       return config_types.Path(config_types.NamedBasePath(name))
     raise KeyError('Unknown path: %s' % name) # pragma: no cover
 
   def __getattr__(self, name):
     # retrieve os.path attributes
     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)