From 02c40d3e2fdda6a1f5be0c834ec1c0002738e9a7 Mon Sep 17 00:00:00 2001
From: Kai Chappell <git@kschappell.com>
Date: Tue, 5 Aug 2025 15:12:34 +0000
Subject: [PATCH] Implement configuration loader

---
 pyproject.toml               |  1 +
 src/py_dvt_ate/app/config.py | 83 +++++++++++++++++++++++++++++++++++-
 2 files changed, 83 insertions(+), 1 deletion(-)

diff --git a/pyproject.toml b/pyproject.toml
index c0bdde5..b6f6921 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -37,6 +37,7 @@ dev = [
     "pytest-asyncio>=0.21",
     "ruff>=0.1",
     "mypy>=1.0",
+    "types-PyYAML>=6.0",
 ]
 
 [project.scripts]
diff --git a/src/py_dvt_ate/app/config.py b/src/py_dvt_ate/app/config.py
index 007a21f..c5e4a47 100644
--- a/src/py_dvt_ate/app/config.py
+++ b/src/py_dvt_ate/app/config.py
@@ -4,8 +4,11 @@ This module defines Pydantic models for all configuration sections.
 Configuration can be loaded from YAML files and validated at runtime.
 """
 
-from typing import Literal
+import os
+from pathlib import Path
+from typing import Any, Literal
 
+import yaml
 from pydantic import BaseModel, Field
 
 
@@ -117,3 +120,81 @@ class AppConfig(BaseModel):
     logging: LoggingConfig = Field(default_factory=LoggingConfig)
     dashboard: DashboardConfig = Field(default_factory=DashboardConfig)
     api: APIConfig = Field(default_factory=APIConfig)
+
+
+def _apply_env_overrides(config_dict: dict[str, Any]) -> None:
+    """Apply environment variable overrides to config dictionary.
+
+    Environment variables follow the pattern: PYDVTATE__{SECTION}__{KEY}
+    For nested keys, use double underscores: PYDVTATE__{SECTION}__{SUBSECTION}__{KEY}
+
+    Examples:
+        PYDVTATE__INSTRUMENTS__BACKEND=pyvisa
+        PYDVTATE__PHYSICS__UPDATE_RATE_HZ=50.0
+        PYDVTATE__SIMULATOR__HOST=192.168.1.100
+    """
+    prefix = "PYDVTATE__"
+
+    for env_key, env_value in os.environ.items():
+        if not env_key.startswith(prefix):
+            continue
+
+        # Remove prefix and split into parts
+        key_parts = env_key[len(prefix) :].lower().split("__")
+
+        # Navigate/create nested structure
+        current = config_dict
+        for part in key_parts[:-1]:
+            if part not in current:
+                current[part] = {}
+            current = current[part]
+
+        # Set the final value
+        final_key = key_parts[-1]
+        # Try to parse as YAML to handle types (int, float, bool, etc.)
+        try:
+            current[final_key] = yaml.safe_load(env_value)
+        except yaml.YAMLError:
+            # If parsing fails, use as string
+            current[final_key] = env_value
+
+
+def load_config(config_path: str | Path | None = None) -> AppConfig:
+    """Load configuration from YAML file with environment variable overrides.
+
+    Args:
+        config_path: Path to YAML configuration file. If None, uses defaults only.
+
+    Returns:
+        Validated AppConfig instance.
+
+    Raises:
+        FileNotFoundError: If config_path is provided but does not exist.
+        yaml.YAMLError: If YAML file is malformed.
+        pydantic.ValidationError: If configuration is invalid.
+
+    Environment Variables:
+        Configuration can be overridden using environment variables with the
+        pattern PYDVTATE__{SECTION}__{KEY}. For example:
+            PYDVTATE__INSTRUMENTS__BACKEND=pyvisa
+            PYDVTATE__PHYSICS__UPDATE_RATE_HZ=50.0
+    """
+    # Start with empty dict (will use Pydantic defaults)
+    config_dict: dict[str, Any] = {}
+
+    # Load from YAML file if provided
+    if config_path is not None:
+        path = Path(config_path)
+        if not path.exists():
+            raise FileNotFoundError(f"Configuration file not found: {config_path}")
+
+        with path.open("r") as f:
+            loaded = yaml.safe_load(f)
+            if loaded is not None:
+                config_dict = loaded
+
+    # Apply environment variable overrides
+    _apply_env_overrides(config_dict)
+
+    # Validate and return
+    return AppConfig(**config_dict)