update config util to make preset loading optional, update docs

tests/core/pyspec/eth2spec/config/README.md

@@ -5,16 +5,23 @@ For configuration, see [Configs documentation](../../../../../configs/README.md)
 ## Usage:
 
 ```python
-configs_path = 'configs/'
-
-...
-
 from eth2spec.config import config_util
 from eth2spec.phase0 import spec
 from importlib import reload
-config_util.prepare_config(configs_path, 'mainnet')
+from pathlib import Path
+
+# To load the presets and configurations
+config_util.load_defaults(Path("eth2.0-specs/configs"))  # change path to point to equivalent of specs `configs` dir.
+# After loading the defaults, a config can be chosen: 'mainnet', 'minimal', or custom network config
+config_util.prepare_config('minimal')
+# Alternatively, load a custom testnet config:
+config_util.prepare_config('my_config.yaml')
 # reload spec to make loaded config effective
 reload(spec)
 ```
 
+Note: previously the testnet config files included both preset and runtime-configuration data.
+The new config loader is compatible with this: just run `prepare_config` without loading preset defaults,
+and omit the `PRESET_BASE` from the config.
+
 WARNING: this overwrites globals, make sure to prevent accidental collisions with other usage of the same imported specs package.

tests/core/pyspec/eth2spec/config/config_util.py

@@ -35,14 +35,17 @@ def prepare_config(config_path: Union[Path, BinaryIO, TextIO, Literal['mainnet']
         conf_data = deepcopy(minimal_config_data)
     else:
         conf_data = load_config_file(config_path)
-    # Check the configured preset
-    base = conf_data['PRESET_BASE']
-    if base not in ('minimal', 'mainnet'):
-        raise Exception(f"unknown PRESET_BASE: {base}")
     # Apply configuration if everything checks out
     global config
-    config = deepcopy(mainnet_preset_data if base == 'mainnet' else minimal_preset_data)
-    config.update(conf_data)
+    if 'PRESET_BASE' in conf_data:
+        # Check the configured preset
+        base = conf_data['PRESET_BASE']
+        if base not in ('minimal', 'mainnet'):
+            raise Exception(f"unknown PRESET_BASE: {base}")
+        config = deepcopy(mainnet_preset_data if base == 'mainnet' else minimal_preset_data)
+        config.update(conf_data)
+    else:
+        config = conf_data
 
 
 def parse_config_vars(conf: Dict[str, Any]) -> Dict[str, Any]: