layered-yaml-attrdict-config 12.07.1

Basic syntax

Idea is the same as with yaml.load() to load YAML configuration file like this one:

core:
  connection:
    # twisted endpoint syntax, see twisted.internet.endpoints.html#clientFromString
    endpoint: tcp:host=example.com:port=6667
    nickname: testbot
    reconnect:
      maxDelay: 30
  xattr_emulation: /tmp/xattr.db

But when you use resulting nested-dicts in code, consider the difference between config['core']['connection']['reconnect']['maxDelay'] and config.core.connection.reconnect.maxDelay.

Python dicts support only the first syntax, this module supports both. Assigning values through attributes is also possible.

Recursive updates (inheritance)

I find it useful to have default parameters specified in the same format as any configurable overrides to them - simple yaml file.

So consider this use-case:

import lya
cfg = lya.AttrDict.from_yaml('default.yaml')
for path in sys.argv[1:]: cfg.update_yaml(path)
cfg.dump(sys.stdout)

(there is also AttrDict.update_dict method for recursive updates from dict)

With default configuration file from the previous section shipped along with the package as “default.yaml”, you can have simple override like:

core:
  connection:
    endpoint: ssl:host=some.local.host:port=6697

And above code will result in the following config (which will be dumped as nicely-formatted yaml, as presented below):

core:
  connection:
    endpoint: ssl:host=some.local.host:port=6697
    nickname: testbot
    reconnect:
      maxDelay: 30
  xattr_emulation: /tmp/xattr.db

Rebase

Similar to the above, but reversed, so result presented above can be produced by taking some arbitrary configuration (AttrDict) and rebasing it on top of some other (base) config:

import lya
base = lya.AttrDict.from_yaml('default.yaml')
for path in sys.argv[1:]:
  cfg.rebase(base)
  print 'Config:', path
  cfg.dump(sys.stdout)

Useful to fill-in default values for similar configuration parts (e.g. configuration for each module or component).

Key ordering

Keys in python dictionaries are unordered and by default, yaml module loses any ordering of keys in yaml dicts as well.

Strictly speaking, this is correct processing of YAML, but for most cases it is inconvenient when instead of clear section like this one:

processing_order:
  receive_test:
    name: '#bot-central'
    server: testserver
  important_filter: '^important:'
  announce: '#important-news'
  debug_filter: '\(debug message\)'
  feedback: botmaster

…you have to resort to putting all the keys that need ordering into a list just to preserve ordering.

Especially annoying if you have to access these sections by key afterwards (and they should be unique) or you need to override some of the sections later, so list wrapper becomes completely artificial as it have to be converted into OrderedDict anyway.

YAML files, parsed from AttrDict.from_yaml and AttrDict.update_yaml methods have key ordering preserved, and AttrDict objects are based on OrderedDict objects, which provide all the features of dict and preserve ordering during the iteration like lists do.

There’s no downside to it - both ordered dicts and lists can be used as usual, if that’s more desirable.

Flattening

Sometimes it’s useful to have nested configuration (like presented above) to be represented as flat list of key-value pairs.

Example usage can be storage of the configuration tree in a simple k-v database (like berkdb) or comparison of configuration objects - ordered flat lists can be easily processed by the “diff” command, tested for equality or hashed.

That is easy to do via AttrDict.flatten() method, producing (from config above) a list like this one:

- (core, connection, endpoint): ssl:host=some.local.host:port=6697
- (core, connection, nickname): testbot
- (core, connection, reconnect, maxDelay): 30
- (core, xattr_emulation): /tmp/xattr.db

Resulting list contains 2-value tuples - key tuple, containing the full path of the value and the value object itself.