add native envsubst

mcp_bridge/config/__init__.py

@@ -1,3 +1,4 @@
+from mcp_bridge.config.env_subst import substitute_env_vars
 from mcp_bridge.config.initial import initial_settings
 from mcp_bridge.config.final import Settings
 from typing import Any, Callable
@@ -38,6 +39,8 @@
     for cfg in configs:
         always_merger.merge(result, cfg)
 
+    result = substitute_env_vars(result)
+
     # build the config
     try:
         config = Settings(**result)

mcp_bridge/config/env_subst.py

@@ -0,0 +1,31 @@
+from string import Template
+from typing import Any
+import os
+
+from loguru import logger
+
+
+def substitute_env_vars(config: Any, env: dict[str, str] | None = None) -> Any:
+    """Substitute environment variables in a configuration object."""
+
+    # copy the environment if it is not provided
+    if env is None:
+        env = os.environ.copy()
+
+    assert env is not None, "env is None"  # the guard should have caught this
+
+    # handle strings
+    if isinstance(config, str):
+        return Template(config).safe_substitute(env)
+
+    # handle other types
+    elif isinstance(config, dict):
+        return {
+            k: substitute_env_vars(v, env) for k, v in config.items() if v is not None
+        }
+
+    # handle lists
+    elif isinstance(config, list):
+        return [substitute_env_vars(v, env) for v in config]
+
+    return config