Design of lsst.pex.config

This page is a collection of notes describing the design of lsst.pex.config and the pex_config package in general.

Design goals

  • Enable configuration of plug-in algorithms provided at runtime.
  • Allow setting of one Field to affect the values and the validation of others.
  • Collocate the Config definition with the code using the Config.
  • Provide a “Pythonic” interface.
  • Record the file and line of Field instance definitions and all changes to Field instances, including setting default values.
  • Set defaults before overriding with user-specified values.
  • Support parameters with no (nonexistent) values, including overriding existing default values.
  • Enable closely-related Config objects to be represented efficiently, with a minimum of duplication.
  • Have all user-modifiable Config instances be part of a hierarchical tree.
  • Validate the contents of Field instances as soon as possible.
  • Be able to “freeze” a Config instance to make it read-only.
  • Be able to persist a Config instance to a file and restore it identically.
  • Allow C++ control objects to be created from Config objects, with documentation and validation specified exactly once.
  • Support lists of parameter values.

Relationship to pex_policy

The lsst.pex.Policy and lsst.pex.PolicyDictionary classes in the pex_policy package provided many of the features of pex_config in C++ and, via SWIG wrapping, Python. pex_config (which contains the lsst.pex.config module) was developed to provide additional features and remove some shortcomings.

pex_policy is being replaced with pex_config in the LSST Data Management System codebase. To aid the transition, the makePolicy a utility function converts a Config to a Policy.


Config uses a metaclass to record the Field attributes within each Config object in an internal dictionary. The storage and history for the fields is also maintained in the Config object, not the Field instance itself. This allows Field classes to be easily inherited.