Fix RuntimeError: OrderedDict mutated during iteration in _fixup_self_references

[haobibo]

Summary

This PR fixes a RuntimeError: OrderedDict mutated during iteration exception that occurs when resolving self-referential optional substitutions under Python 3.13+ (or when key deletion is triggered during substitution resolution).

Cause

In HOCON configs, self-referential environment overrides are commonly defined as follows:

APP_MODULE = "Aloha"
APP_MODULE = ${?APP_MODULE}

When parsing such configurations via include directives, the value history is flattened, and pyhocon tries to resolve ${?APP_MODULE} as a new optional self-reference. If the APP_MODULE environment variable is not defined, ConfigParser._fixup_self_references will substitute it with None via cls._do_substitute(substitution, None).

This operation deletes the key from the ConfigTree (which inherits from collections.OrderedDict ). Since the parser iterates directly over the ConfigTree object ( for key in config ), mutating its size by deleting the key throws RuntimeError: OrderedDict mutated during iteration (particularly strict under Python 3.13+).

Solution

Wrap the iteration in list(...) to iterate over a static snapshot of the keys:

• Before for key in config:
• After for key in list(config):

This decouples the dictionary iteration from any key mutations/deletions performed during the self-reference resolution phase.

[haobibo]

@darthbear Could you please kindly take a review?