recipe engine: require all path bases to be classes

recipe_modules/path/example.py

 # Copyright 2014 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.
 
 DEPS = [
   'path',
   'platform',
   'properties',
   'step',
 ]
 
 from recipe_engine.config_types import Path
 
 def RunSteps(api):
   api.step('step1', ['/bin/echo', str(api.path['tmp_base'].join('foo'))])
 
   # module.resource(...) demo.
   api.step('print resource',
            ['echo', api.path.resource('dir', 'file.py')])
 
   # module.package_repo_resource() demo.
   api.step('print package dir',
            ['echo', api.path.package_repo_resource('dir', 'file.py')])
 
-  # Global dynamic paths (see config.py example for declaration):
-  dynamic_path = Path('[CHECKOUT]', 'jerky')
-
   assert 'checkout' not in api.path
   api.path['checkout'] = api.path['tmp_base'].join('checkout')
   assert 'checkout' in api.path
 
+  # Global dynamic paths (see config.py example for declaration):
+  dynamic_path = api.path['checkout'].join('jerky')
   api.step('checkout path', ['/bin/echo', dynamic_path])
 
   # Methods from python os.path are available via api.path.
   # For testing, we asserted that this file existed in the test description
   # below.
   assert api.path.exists(api.path['tmp_base'])
 
   temp_dir = api.path.mkdtemp('kawaab')
   assert api.path.exists(temp_dir)
 
@@ skipping 17 matching lines @@
     # reason. TODO(phajdan.jr) remove it.
     yield (api.test('%s_swarming' % platform) +
            api.platform.name(platform) +
            api.properties(path_config='swarming') +
            api.path.exists(api.path['tmp_base']))
 
     yield (api.test('%s_kitchen' % platform) +
            api.platform.name(platform) +
            api.properties(path_config='kitchen') +
            api.path.exists(api.path['tmp_base']))