Source code for google.appengine.api.lib_config

#!/usr/bin/env python
#
# Copyright 2007 Google Inc.
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# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
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#     http://www.apache.org/licenses/LICENSE-2.0
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# distributed under the License is distributed on an "AS IS" BASIS,
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"""A mechanism for library configuration.

Whenever App Engine library code needs a user-configurable value, it should use
the following protocol:
    1. Pick a prefix unique to the library module, for example: `mylib`.
    2. Call `lib_config.register(prefix, mapping)` with that prefix as the first
       argument and a dict mapping suffixes to default functions as the second.
    3. The `register()` function returns a configuration handle that is unique
       to this prefix. The configuration handle object has attributes that
       correspond to each of the suffixes given in the mapping. Call these
       functions to access the user's configuration value.  If the user didn't
       configure a function, the default function from the mapping is called
       instead.
    4. Document the function name and its signature and semantics.

Users that want to provide configuration values should create a module named
`appengine_config.py` in the top-level directory of their application and define
functions as documented by various App Engine library components in that module.
To change the configuration, edit the file and re-deploy the application. When
using the SDK, no redeployment is required; the development server will pick up
the changes the next time it handles a request.

Third party libraries can also use this mechanism. For casual use, calling the
`register()` method with a unique prefix is acceptable. For more complex
libraries, however, you should instantiate a new `LibConfigRegistry` instance
that uses a different module name.


Example `appengine_config.py` file::

    from somewhere import MyMiddleWareClass

    def mylib_add_middleware(app):
      app = MyMiddleWareClass(app)
      return app


Example library use::

    from google.appengine.api import lib_config

    config_handle = lib_config.register(
        'mylib',
        {'add_middleware': lambda app: app})

    def add_middleware(app):
      return config_handle.add_middleware(app)
"""



__all__ = ['DEFAULT_MODNAME',
           'LibConfigRegistry',
           'ConfigHandle',
           'register',
           'main',
           ]


import logging
import os
import sys
import threading


DEFAULT_MODNAME = 'appengine_config'





[docs]class LibConfigRegistry(object): """A registry containing library configuration values.""" def __init__(self, modname): """Constructor. Args: modname: The module name to be imported. Note: the actual import of this module is deferred until the first time a configuration value is requested through attribute access on a ConfigHandle instance. """ self._modname = modname self._registrations = {} self._module = None self._lock = threading.RLock()
[docs] def register(self, prefix, mapping): """Registers a set of configuration names. Args: prefix: A shared prefix for the configuration names being registered. If the prefix doesn't end in `_`, that character is appended. mapping: A dict that maps suffix strings to default values. Returns: A `ConfigHandle` instance. You can re-register the same prefix: the mappings are merged, and for duplicate suffixes, the most recent registration is used. """ if not prefix.endswith('_'): prefix += '_' self._lock.acquire() try: handle = self._registrations.get(prefix) if handle is None: handle = ConfigHandle(prefix, self) self._registrations[prefix] = handle finally: self._lock.release() handle._update_defaults(mapping) return handle
[docs] def initialize(self, import_func=__import__): """Tries to import the configuration module if it is not already imported. This function always sets `self._module` to a value that is not `None`; either the imported module (if it was imported successfully) or a dummy `object()` instance (if an `ImportError` was raised) is used. Other exceptions are not caught. When a dummy instance is used, the instance is also put in `sys.modules`. This usage allows us to detect when `sys.modules` was changed (as `dev_appserver.py` does when it notices source code changes) and retries the `__import__` in that case, while skipping it (for speed) if nothing has changed. Args: import_func: Used for dependency injection. """ self._lock.acquire() try: if (self._module is not None and self._module is sys.modules.get(self._modname)): return try: import_func(self._modname) except ImportError, err: if str(err) != 'No module named %s' % self._modname: raise self._module = object() sys.modules[self._modname] = self._module else: self._module = sys.modules[self._modname] finally: self._lock.release()
[docs] def reset(self): """Drops the imported configuration module. If the configuration module has not been imported, no operation occurs, and the next operation takes place. """ self._lock.acquire() try: if self._module is None: return self._module = None handles = self._registrations.values() finally: self._lock.release() for handle in handles: handle._clear_cache()
def _pairs(self, prefix): """Generates `(key, value)` pairs from the config module matching prefix. Args: prefix: A prefix string ending in `_`, for example: `mylib_`. Yields: `(key, value)` pairs, where `key` is the configuration name with the prefix removed, and `value` is the corresponding value. """ self._lock.acquire() try: mapping = getattr(self._module, '__dict__', None) if not mapping: return items = mapping.items() finally: self._lock.release() nskip = len(prefix) for key, value in items: if key.startswith(prefix): yield key[nskip:], value def _dump(self): """Prints information about all registrations to stdout.""" self.initialize() handles = [] self._lock.acquire() try: if not hasattr(self._module, '__dict__'): print 'Module %s.py does not exist.' % self._modname elif not self._registrations: print 'No registrations for %s.py.' % self._modname else: print 'Registrations in %s.py:' % self._modname print '-'*40 handles = self._registrations.items() finally: self._lock.release() for _, handle in sorted(handles): handle._dump()
[docs]class ConfigHandle(object): """A set of configuration for a single library module or package. Public attributes of instances of this class are configuration values. Attributes are dynamically computed (in `__getattr__()`) and cached as regular instance attributes. """ _initialized = False def __init__(self, prefix, registry): """Constructor. Args: prefix: A shared prefix for the configuration names being registered. It must end in `_`. This requirement is enforced by `LibConfigRegistry`. registry: A `LibConfigRegistry` instance. """ assert prefix.endswith('_') self._prefix = prefix self._defaults = {} self._overrides = {} self._registry = registry self._lock = threading.RLock() def _update_defaults(self, mapping): """Updates the default mappings. Args: mapping: A dict mapping suffix strings to default values. """ self._lock.acquire() try: for key, value in mapping.iteritems(): if key.startswith('__') and key.endswith('__'): continue self._defaults[key] = value if self._initialized: self._update_configs() finally: self._lock.release() def _update_configs(self): """Updates the configuration values. This clears the cached values, initializes the registry, and loads the configuration values from the config module. """ self._lock.acquire() try: if self._initialized: self._clear_cache() self._registry.initialize() for key, value in self._registry._pairs(self._prefix): if key not in self._defaults: logging.warn('Configuration "%s" not recognized', self._prefix + key) else: self._overrides[key] = value self._initialized = True finally: self._lock.release() def _clear_cache(self): """Clears the cached values.""" self._lock.acquire() try: self._initialized = False for key in self._defaults: self._overrides.pop(key, None) try: delattr(self, key) except AttributeError: pass finally: self._lock.release() def _dump(self): """Prints information about this set of registrations to stdout.""" self._lock.acquire() try: print 'Prefix %s:' % self._prefix if self._overrides: print ' Overrides:' for key in sorted(self._overrides): print ' %s = %r' % (key, self._overrides[key]) else: print ' No overrides' if self._defaults: print ' Defaults:' for key in sorted(self._defaults): print ' %s = %r' % (key, self._defaults[key]) else: print ' No defaults' print '-'*40 finally: self._lock.release() def __getattr__(self, suffix): """Dynamic attribute access. Args: suffix: The attribute name. Returns: A configuration values. Raises: AttributeError: If the suffix is not a registered suffix. The first time an attribute is referenced, this method is invoked. The value returned is taken either from the config module or from the registered default. """ self._lock.acquire() try: if not self._initialized: self._update_configs() if suffix in self._overrides: value = self._overrides[suffix] elif suffix in self._defaults: value = self._defaults[suffix] else: raise AttributeError(suffix) setattr(self, suffix, value) return value finally: self._lock.release()
_default_registry = LibConfigRegistry(DEFAULT_MODNAME)
[docs]def register(prefix, mapping): """Register a set of configurations with the default config module. Args: prefix: A shared prefix for the configuration names being registered. If the prefix doesn't end in `_`, that character is appended. mapping: A dict mapping suffix strings to default values. Returns: A `ConfigHandle` instance. """ return _default_registry.register(prefix, mapping)
[docs]def main(): """Dumps the configuration, using a CGI-style request handler. Put this in your `app.yaml` file to enable (you can pick any URL):: - url: /lib_config script: $PYTHON_LIB/google/appengine/api/lib_config.py Note: Unless you are using the SDK, you must be an administrator to use this function. """ if not os.getenv('SERVER_SOFTWARE', '').startswith('Dev'): from google.appengine.api import users if not users.is_current_user_admin(): if users.get_current_user() is None: print 'Status: 302' print 'Location:', users.create_login_url(os.getenv('PATH_INFO', '')) else: print 'Status: 403' print print 'Forbidden' return print 'Content-type: text/plain' print _default_registry._dump()
if __name__ == '__main__': main()

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