Utility object for expanding a dict of configuration template strings

config_resolver.py

class ConfigResolver(object):

    def __init__(self, **kwargs):
        self.config_templates = kwargs
        self.config = resolve(copy.copy(kwargs))

    def __getattr__(self, attr):
        return self.config[attr]

    def __getitem__(self, item):
        return self.config[item]

    def __unicode__(self):
        return unicode(self.config)
 

def resolve(config_templates, initial_config=None):
    config = initial_config or {}
    while True:
        error_keys = set()
        resolved_one_or_more = False
        for name, template in config_templates.items():
            try:
                config[name] = template.format(**config)
                del config_templates[name]
                resolved_one_or_more = True
            except KeyError as e:
                error_keys.add(e.message)

        if len(config_templates) == 0:
            break

        if not resolved_one_or_more:
            raise Exception('Failed to resolve all references')
    return config

usage.md

Basic configuration

The basic resolution mechanism just loops over a dict's key/value pairs, tries to fill in their format strings using the established configuration, and catches errors to retry in a later round of resolution:

template_dict = {
  'app_root': '/usr/local/myapp',
  'log': '{app_root}/logs',
  'uwsgi_log': '{log}/uwsgi.log',
  'deploy': '{app_root}/.deploy/{commit_sha}',
  'version_src': '{deploy}/src',
  'src_link': '{app_root}/src',
  'version_env': '{deploy}/env',
  'env_link': '{app_root}/env',
  'commit_sha': 'a1b2c3',
}

resolve(template_dict)
>>> {
  'app_root': '/usr/local/myapp',
  'log': '/usr/local/myapp/logs',
  'uwsgi_log': '/usr/local/myapp/logs/uwsgi.log',
  'deploy': '/usr/local/myapp/.deploy/a1b2c3',
  'version_src': '/usr/local/myapp/.deploy/a1b2c3/src',
  'src_link': '/usr/local/myapp/src',
  'version_env': '/usr/local/myapp/.deploy/a1b2c3/env',
  'env_link': '/usr/local/myapp/env',
  'commit_sha': 'a1b2c3',
}

Configuration object

The configuration wrapper object provides both attr- and item-based lookups on the resolved configuration, and takes **kwargs rather than a dict as configuration:

config = ConfigResolver(template_dict)
 
#  or, equivalently...
config = ConfigResolver(
  app_root='/usr/local/myapp',
  log='{app_root}/logs',
  uwsgi_log='{log}/uwsgi.log',
  deploy='{app_root}/.deploy/{commit_sha}',
  version_src='{deploy}/src',
  src_link='{app_root}/src',
  version_env='{deploy}/env',
  env_link='{app_root}/env',
  commit_sha='a1b2c3',
)

config['version_src']
>>> '/usr/local/myapp/.deploy/a1b2c3'

config.uwsgi_log
>>> '/usr/local/myapp/logs/uwsgi.log'