RuntimeError: dictionary changed size during iteration in OmegaConf.resolve() on Python 3.12+

[Moep90]

Bug Report

Description

OmegaConf.resolve() raises RuntimeError: dictionary changed size during iteration on Python 3.12+ when a custom resolver returns a DictConfig (Container).

Root Cause

_resolve() in omegaconf/_impl.py iterates over a live dict_keys view:

for k in cfg.keys():
    _resolve_container_value(cfg, k)

Inside _resolve_container_value, when a resolver returns a Container and the current node is a ValueNode, the result is assigned back:

if isinstance(resolved, Container) and isinstance(node, ValueNode):
    cfg[key] = resolved

This assignment modifies the underlying dict while the cfg.keys() view is being iterated. Python 3.12+ introduced stricter checks that raise RuntimeError on structural dict modifications during iteration.

Minimal Reproduction

from omegaconf import OmegaConf, ListMergeMode

def merge(*args):
    return OmegaConf.merge(*args, list_merge_mode=ListMergeMode.EXTEND)

OmegaConf.register_new_resolver("merge", merge, replace=True)

cfg = OmegaConf.create({
    "base": {"x": 1, "y": 2},
    "extra": {"z": 3},
    "merged": "${merge:${base},${extra}}",
})

OmegaConf.resolve(cfg)  # RuntimeError: dictionary changed size during iteration

Traceback

  File ".../omegaconf/_impl.py", line 52, in _resolve
    for k in cfg.keys():
             ~~~~~~~~^^
RuntimeError: dictionary changed size during iteration

Environment

• Python 3.12+ (confirmed on 3.12 and 3.14)
• omegaconf 2.4.0.dev4
• Works on Python ≤ 3.11

Fix

Snapshot the keys before iterating:

for k in list(cfg.keys()):
    _resolve_container_value(cfg, k)

A PR with this fix and a regression test is attached.