Write a custom resolver

A resolver is a Python callable used in a resolver call. Register it once with a name, then use that name in ${name:arguments}:

>>> from omegaconf import OmegaConf
>>> OmegaConf.register_new_resolver("plus_ten_docs", lambda value: value + 10)
>>> cfg = OmegaConf.create({"result": "${plus_ten_docs:32}"})
>>> cfg.result
42

Resolver arguments are parsed from the interpolation expression. The resolver runs when the node is accessed. Nested interpolation lets an argument come from another config value, and a resolver may return a scalar, container, or config node. Names may be namespaced (such as myapp.add), and an interpolation may even select part of a resolver name.

If the callable declares keyword-only _parent_ or _root_ parameters, OmegaConf supplies the containing node or config root. This is useful when a resolver needs to inspect neighboring values without requiring them as explicit interpolation arguments.

>>> def add_neighbors(left, right, *, _parent_):
...     return _parent_.get(left, 0) + _parent_.get(right, 0)
>>> OmegaConf.register_new_resolver("add_neighbors_docs", add_neighbors)
>>> cfg = OmegaConf.create({
...     "group": {
...         "a": 1,
...         "b": 2,
...         "total": "${add_neighbors_docs:a,b}",
...         "with_absent": "${add_neighbors_docs:a,absent}",
...     },
... })
>>> cfg.group.total, cfg.group.with_absent
(3, 1)

The name absent is passed as text; the resolver handles its absence through _parent_. A missing nested argument, by contrast, fails before the resolver is called.

Replace or cache a resolver

Registering a name that already exists raises ValueError. Pass replace=True when deliberately replacing it. use_cache=True reuses a result for the same literal argument strings, even if a nested interpolation would now resolve to a different value; leave caching off when the function depends on changing external state.

>>> OmegaConf.register_new_resolver(
...     "cached_docs", lambda value: value * 2, use_cache=True
... )
>>> cfg = OmegaConf.create({
...     "source": 2,
...     "result": "${cached_docs:${source}}",
... })
>>> cfg.result
4
>>> cfg.source = 3
>>> cfg.result  # the same literal argument expression uses the cache
4

Remove resolvers

Clear one

OmegaConf.clear_resolver(name) returns True if it removed a registration and False if the name was absent. It can remove a built-in, so check the name before calling it.

>>> OmegaConf.register_new_resolver("temporary_docs", lambda: 1)
>>> OmegaConf.clear_resolver("temporary_docs")
True
>>> OmegaConf.has_resolver("temporary_docs")
False

Clear all

OmegaConf.clear_resolvers() removes custom registrations but retains the built-ins:

>>> OmegaConf.clear_resolvers()
>>> OmegaConf.has_resolver("oc.env")
True

The 2.4 resolver guide describes the new OmegaConf.register_resolver() API and annotation validation. See built-in resolvers for oc.env, oc.select, and other resolvers provided by OmegaConf.