Python 2 Module Configuration

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:

namespace_manager
  • default_namespace_for_request() (default returns None)

appstats

datastore_admin

  • BASE_PATH (default '/_ah/datastore_admin')
  • MAPREDUCE_PATH (default '/_ah/mapreduce')
  • CLEANUP_MAPREDUCE_STATE (default True)

remoteapi

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.