Clarify in docs how runtime parameter resolution works

RELEASE.md

@@ -10,6 +10,7 @@
 ## Breaking changes to the API
 
 ## Documentation changes
+* Add clarifications in docs explaining how runtime parameter resolution works.
 
 ## Community contributions
 

docs/source/configuration/advanced_configuration.md

@@ -322,7 +322,8 @@ Note that you can only use the resolver in `credentials.yml` and not in catalog
 ```
 
 ### How to change the merge strategy used by `OmegaConfigLoader`
-By default, `OmegaConfigLoader` merges configuration [in different environments](configuration_basics.md#configuration-environments) in a destructive way. This means that whatever configuration resides in your overriding environment (`local` by default) takes precedence when the same top-level key is present in the base and overriding environment. Any configuration for that key **besides that given in the overriding environment** is discarded.
+By default, `OmegaConfigLoader` merges configuration [in different environments](configuration_basics.md#configuration-environments) as well as runtime parameters in a destructive way. This means that whatever configuration resides in your overriding environment (`local` by default) takes precedence when the same top-level key is present in the base and overriding environment. Any configuration for that key **besides that given in the overriding environment** is discarded.
+The same behaviour applies to runtime parameters overriding any configuration in the `base` environment.
 You can change the merge strategy for each configuration type in your project's `src/<package_name>/settings.py`. The accepted merging strategies are `soft` and `destructive`.
 
 ```python

docs/source/configuration/parameters.md

@@ -117,7 +117,8 @@ Each key-value pair is split on the first equals sign. The following example is
 ```bash
 kedro run --params=param_key1=value1,param_key2=2.0
 ```
-Values provided in the CLI take precedence and overwrite parameters specified in configuration files.
+Values provided in the CLI take precedence and overwrite parameters specified in configuration files. By default, runtime parameters get merged destructively, meaning that any configuration for that key **besides that given in the runtime parameters** is discarded.
+[This section describes how to change the merging strategy](#how-to-change-the-merge-strategy-used-by-omegaconfigloader).
 
 * Parameter keys are _always_ treated as strings.
 * Parameter values are converted to a float or an integer number if the corresponding conversion succeeds; otherwise, they are also treated as string.

tests/config/test_omegaconf_config.py

@@ -1068,6 +1068,89 @@ def test_runtime_params_resolution(self, tmp_path):
         # runtime params are resolved correctly in catalog
         assert conf["catalog"]["companies"]["type"] == runtime_params["dataset"]["type"]
 
+    def test_runtime_params_resolution_with_soft_merge_base_env(self, tmp_path):
+        """Test that runtime_params get softly merged with the base environment when soft merge is set
+        for parameter merge"""
+        base_params = tmp_path / _BASE_ENV / "parameters.yml"
+        prod_params = tmp_path / "prod" / "parameters.yml"
+        runtime_params = {
+            "aaa": {
+                "bbb": {
+                    "abb": "2011-11-11",
+                }
+            }
+        }
+        param_config = {
+            "aaa": {
+                "bbb": {
+                    "aba": "2023-11-01",
+                    "abb": "2023-11-01",
+                    "abc": 14,
+                }
+            },
+            "xyz": {"asdf": 123123},
+        }
+        prod_param_config = {"def": {"gg": 123}}
+        _write_yaml(base_params, param_config)
+        _write_yaml(prod_params, prod_param_config)
+        conf = OmegaConfigLoader(
+            tmp_path,
+            base_env=_BASE_ENV,
+            default_run_env="prod",
+            runtime_params=runtime_params,
+            merge_strategy={"parameters": "soft"},
+        )
+
+        expected_parameters = {
+            "aaa": {"bbb": {"aba": "2023-11-01", "abb": "2011-11-11", "abc": 14}},
+            "xyz": {"asdf": 123123},
+            "def": {"gg": 123},
+        }
+
+        # runtime parameters are resolved correctly across parameter files from different environments
+        assert conf["parameters"] == expected_parameters
+
+    def test_runtime_params_resolution_default_run_env(self, tmp_path):
+        """Test that runtime_params overwrite merge with the default run environment"""
+        base_params = tmp_path / _BASE_ENV / "parameters.yml"
+        prod_params = tmp_path / "prod" / "parameters.yml"
+        runtime_params = {"data_shift": 3}
+        param_config = {
+            "model_options": {
+                "test_size": 0.2,
+                "random_state": 3,
+                "features": ["engines"],
+            }
+        }
+        prod_param_config = {
+            "model_options": {
+                "test_size": 0.2,
+                "random_state": 3,
+                "features": ["engines"],
+            },
+            "data_shift": 1,
+        }
+        _write_yaml(base_params, param_config)
+        _write_yaml(prod_params, prod_param_config)
+        conf = OmegaConfigLoader(
+            tmp_path,
+            base_env=_BASE_ENV,
+            default_run_env="prod",
+            runtime_params=runtime_params,
+        )
+
+        expected_parameters = {
+            "model_options": {
+                "test_size": 0.2,
+                "random_state": 3,
+                "features": ["engines"],
+            },
+            "data_shift": 3,
+        }
+
+        # runtime parameters are resolved correctly across parameter files from different environments
+        assert conf["parameters"] == expected_parameters
+
     def test_runtime_params_missing_default(self, tmp_path):
         base_params = tmp_path / _BASE_ENV / "parameters.yml"
         runtime_params = {