The appengine_config.py
file is a configuration file that provides you the
ability to specify the
installation folder for libraries and provide your own values for constants and
"hook functions" for some of the Python modules in the google.appengine packages.
Specifying your own values can change the default behavior of the
related App Engine services based on the application's needs. You define this file alongside your
app.yaml
configuration
file and deploy it with the rest of your app's code.
Configuring Python modules with appengine_config.py
Several Python modules in
App Engine are configurable using appengine_config.py
.
To customize the Python modules of your services,
you create a new appengine_config.py
file in the root directory
of that service. To use this file, you need to define only those constants or
hook functions you wish to override. You then run gcloud app
deploy
from the directory where the app.yaml
file is
located to redeploy your app along with the new
appengine_config.py
file. The constants and hook functions that
you defined will then be used by those App Engine services internally.
To override a constant, prefix the constant's name with the Python module
name and an underscore, then assign a value. For example, to edit overrides in
appstats, you can
define the value of KEY_PREFIX
appstats_KEY_PREFIX = '__my_custom_prefix__'
Naming of overridden hook functions is similar in other Python modules. For
example, in
namespace_manager, you can override the hook function
default_namespace_for_request
in appengine_config.py
as follows:
import os def namespace_manager_default_namespace_for_request(): return os.environ.get('HTTP_HOST', '')
Configurable Python modules in App Engine
The Python modules listed below are configurable using
appengine_config.py
. By convention, hook functions are lowercase
and constants are uppercase:
-
default_namespace_for_request()
(default returnsNone
)
- see
class ConfigDefaults
in appengine/ext/appstats/recording.py - see example in appengine/ext/appstats/sample_appengine_config.py
BASE_PATH
(default'/_ah/datastore_admin'
)MAPREDUCE_PATH
(default'/_ah/mapreduce'
)CLEANUP_MAPREDUCE_STATE
(defaultTrue
)
CUSTOM_ENVIRONMENT_AUTHENTICATION
- see
class ConfigDefaults
in appengine/ext/remote_api/handler.py
Configuring your own Python modules with lib_config
App Engine also allows you to configure your own Python modules with constants
and hook functions defined in appengine_config.py
. The
lib_config.register()
function allows you to both register the
names of the user-overridable constants and hooks, and to define sensible
defaults in case the users don't wish to override them. Internally,
lib_config.register()
attempts to import
appengine_config
. If successful, it replaces the defaults of the
specified Python modules with those defined in
appengine_config.py
.
Example usage in my_module.py
:
from google.appengine.api import lib_config def _hook_function1_default(): return 'baz' _config = lib_config.register('my_module', {'CONSTANT1': 'foo', 'CONSTANT2': 'bar', 'hook_function1': _hook_function1_default})
Now you can access a user's constants as:
_config.CONSTANT1 _config.CONSTANT2
and call their hook function as:
_config.hook_function1()
Some programmers like to group their defaults into a class:
class _ConfigDefaults(object): CONSTANT1 = 'foo' CONSTANT2 = 'bar' def hook_function1(): return 'baz' _config = lib_config.register('my_module', _ConfigDefaults.__dict__)
In order to override your defaults, a user could define in
appengine_config.py
:
my_module_CONSTANT1 = 'foofoo' my_module_hook_function1 = lambda: 'bazbaz'
As a result, in my_module.py
, the following will be true:
_config.CONSTANT1
is now'foofoo'
_config.CONSTANT2
remains'bar'
_config.hook_function1()
returns'bazbaz'
The user overrides are available to my_module.py
immediately
after lib_config.register()
returns.