From c1c5c0b93ecc93815afb3554a2812fb22b50d9c4 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Wed, 7 May 2025 18:51:40 -0600
Subject: [PATCH] Apply resolve paths to path values in `config` (#3400)

---
 docs/source/usersguide/data.rst | 3 ++-
 openmc/config.py                | 2 +-
 tests/unit_tests/test_config.py | 7 +++++++
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/docs/source/usersguide/data.rst b/docs/source/usersguide/data.rst
index 2a9cd36dbb..8b2938556b 100644
--- a/docs/source/usersguide/data.rst
+++ b/docs/source/usersguide/data.rst
@@ -30,7 +30,8 @@ responsible for specifying one or more of the following:
 Each of the above files can specified in several ways. In the Python API, a
 :ref:`runtime configuration variable <usersguide_data_runtime>`
 :data:`openmc.config` can be used to specify any of the above and is initialized
-using a set of environment variables.
+using a set of environment variables. Data configuration paths set in
+:data:`openmc.config` will be expanded to absolute paths.
 
 .. _usersguide_data_runtime:
 
diff --git a/openmc/config.py b/openmc/config.py
index ab53ab61b5..56d8a4072d 100644
--- a/openmc/config.py
+++ b/openmc/config.py
@@ -60,7 +60,7 @@ class _Config(MutableMapping):
         return repr(self._mapping)
 
     def _set_path(self, key, value):
-        self._mapping[key] = p = Path(value)
+        self._mapping[key] = p = Path(value).resolve()
         if not p.exists():
             warnings.warn(f"'{value}' does not exist.")
 
diff --git a/tests/unit_tests/test_config.py b/tests/unit_tests/test_config.py
index 9d3f53a740..4e87de4b7e 100644
--- a/tests/unit_tests/test_config.py
+++ b/tests/unit_tests/test_config.py
@@ -1,5 +1,6 @@
 from collections.abc import Mapping
 import os
+from pathlib import Path
 
 import openmc
 import pytest
@@ -34,6 +35,12 @@ def test_config_basics():
     with pytest.raises(KeyError):
         openmc.config['🐖'] = '/like/to/eat/bacon'
 
+    # ensure relative paths are expanded into absolute
+    # paths
+    chain_path = Path('./chain.xml')
+    openmc.config['chain_file'] = chain_path
+    assert openmc.config['chain_file'] == chain_path.resolve()
+
 
 def test_config_patch():
     openmc.config['cross_sections'] = '/path/to/cross_sections.xml'