A YAML based configuration DSL and associated parser/validator.

pip install py-yacc==4.0.0a2



Python implementation of a Yet Another Configuration Compiler.

Why? What about INI, XML, JSON, or plain Python?

The YACC DSL provides an efficient way to specify, document, and construct a typed configuration system that supports common patterns through overlays, environment injection, default, optional, and required variables.

None of the above support this as simply or clearly as the YACC DSL and python configuration builder.


The system is built around three concepts:

  • A YACC Descriptor, authored in a YAML DSL.
  • Overlays which are additive.
  • Output, which represents the assembly of a YACC Descriptor and the Overlays.

YACC Descriptor

The YACC Descriptor borrows heavily from the INI format: a configuration is a collection of sections, with one or more keys within a section.

The YACC Descriptor adds the notion of a Specification, which describes a key:

  • The type
  • A default value (or optional or required)
  • Documenation
  • Examples
  • Deprecated (flag)
  • Sensitive


  port: !spec
    description: Port to accept general traffic.
    type: !!int "0"
    value: 80
  debug: !spec
    description: Is debug mode enabled.
    type: !!bool "0"
    value: false
  public-name: !spec
    description: DNS name for routing public traffic (e.g. redirects).
    type: !!str ""
    value: !environment HOSTNAME
  ssl-private-key: !spec
    sensitive: true
    type: !!str ""
    value: !required        


An overlay is simply a set of key-level overrides, where last takes precedence.

Overlay as JSON

This can be provided as a YAML document (or equivalent JSON):

  port: 9999

Overlay as ENV variable

Via a key the environment (the prefix is configurable):



After merging the descriptor and all overlays via the algorithm described below the compiled configuration is a Dict[section, Dict[key, value]]

This can be injected into a running python process or output via the CLL into formats consumable by other programs (INI, SH, Make, JSON).

Resolution Algorithm

Overlay Assembly

desc_object = yml.load(open(file).read()) #: :type: Dict[str, Dict[str, ValueSpec]]

resolver = YaccResolver(desc_object)

1. Parse the descriptor and provided overlays.

The descriptor defines the complete set of accepted parameters as defined by ValueSpec objects.

2. Resolve overlays as defined in the environment

default_overlays = [EnvVar("YACC__OVERLAY")]
extra_overlays = os.environ.get("PYACC_RESOLVER__OVERLAYS")
if not extra_overlays:
    extra_overlays = default_overlays
return map(yml.load, filter(None, map(read, extra_overlays)))    

3. Define the environment key prefix ${ENV_PREFIX}

The environment prefix is used to resolve key-specific overlays.

env_prefix = os.environ.get("YACC_RESOLVER__ENV_PREFIX", "PYYACC")

4. Construct an overlay from the environment

  • Values are parsed as YAML values.
  • Section/keys map to:
re.sub(r'[^a-z0-9]', '_', "%s__%s__%s" % (env_prefix, section, key), flags=re.I).upper()

4. Merge down each overlay onto the descriptor.

descriptor.update(*itertools.chain(provided_overlays, extra_overlays, [environment_overlay]))

Final Resolution


1. Iterate over all ValueSpecs

  1. Resolve any values !environment objects.
  2. Validate type against the spec.
  3. Store values and errors.


pyyacc3 -h
Usage: pyyacc3 [options] yaml [yaml ...]

  -h, --help            show this help message and exit
  -v                    Verbose logging output.
  -V                    Very verbose logging output.
  --flat                Flatten into 'section.key': value notation
  -f ARG_FORMAT, --format=ARG_FORMAT
                        Output format: yaml, json, sh, make are supported.
  --no-validate         Disable validation [default: on]
  -o ARG_OUTPUT, --output=ARG_OUTPUT
                        Output destination: path where to write output. If not
                        provided, stdout is used.
                        Prefix for overlays from the environment
                        Name of an overlay to load from the environment