Treating configuration values as templates

A while back I described fassembler and one of the things I liked in it is how the configuration works. It uses a conventional declarative INI-style but also allows arbitrary code, so that defaults can be based on each other.

Here’s a basic example of a default configuration:

port_offset = 10
port = {{int(section.DEFAULT['base_port'])+int(port_offset)}}

Then if another configuration file defines base_port then this will all resolve. You can do this in Python, but you don’t get sections, and you have to define everything in just the right order. So while base_port will probably be defined in a deployment-specific configuration, it has to be defined before these other derivative settings are defined. On the other hand, you want deployment-specific configuration to take precedence… so there’s really no good ordering.

Anyway, the implementation really isn’t that hard. I use Tempita as the templating language because, well, I wrote it, and because it’s simple and appropriate for small strings. For the configuration parsing, ConfigParser will do.

Here’s what the basic code looks like in ConfigParser:

from ConfigParser import ConfigParser
from tempita import Template

class TempitaConfigParser(ConfigParser):

    def _interpolate(self, section, option, rawval, vars):
        ns = _Namespace(self, section, vars)
        tmpl = Template(rawval, name='%s.%s' % (section, option))
        value = tmpl.substitute(ns)
        return value

Actually instead of using tempita.Template, we could just do eval(rawval, {}, ns), it would just require a lot more quoting (every value would have to be a valid Python expression). Either with that or Tempita the implementation of _Namespace will look the same.

Here’s a simple implementation:

from UserDict import DictMixin

class _Namespace(DictMixin):
    def __init__(self, config, section, vars):
        self.config = config
        self.section = section
        self.vars = vars

    def __getitem__(self, key):
        if key == 'section':
            return _Section(self)
        if self.config.has_option(self.section, key):
            return self.config.get(self.section, key)
        if vars and key in self.vars:
            return self.vars[key]
        raise KeyError(key)

   def __setitem__(self, key, value):
       if self.vars is None:
           self.vars = {key: value}
           self.vars[key] = value

We’ve introduced a magic variable section, which is used to refer to other sections. It looks like this:

class _Section(object):
    def __init__(self, namespace):
        self._namespace = namespace

    def __getattr__(self, attr):
        if attr.startswith('_'):
            raise AttributeError(attr)
        return _Namespace(self._namespace.config, attr,     self._namespace.vars)

With these I think you get many of the benefits of using Python code as your configuration format, while still having the benefits of a more declarative approach to configuration, one that allows for forward and backward references.

A full implementation has several more things than I show here, but you can see the full example in my recipes. It also has an example of using INITools instead of ConfigParser to give more accurate filenames and line numbers when there is an exception, while otherwise using the same interface.