Allow [[core.data.variable]] to be split across files

docs/source/using_the_ve/data/data.md

@@ -210,10 +210,9 @@ file="'../../data/xy_dim.nc'"
 var_name="temp"
 ```
 
-**NOTE**: At the moment,
-`core.data.variable` tags cannot be used across multiple toml config files without
-causing `ConfigurationError: Duplicated entries in config files: core.data.variable` to
-be raised. This means that all variables need to be combined in one `config` file.
+You can include `core.data.variable` tags in different files. This can be useful to
+group model-specific data with other model configuration options, and allow
+configuration files to be swapped in a more modular fashion.
 
 To load configuration data , you will typically use the `cfg_paths` argument
 to pass one or more TOML formatted configuration files to create a

tests/core/test_config.py

@@ -135,6 +135,29 @@
             ("b.bb.bbb.bbba.bbbaa",),
             id="conflict_complex",
         ),
+        pytest.param(
+            {"d1": {"d2": [1, 2, 3]}},
+            {"d1": {"d2": [4, 5]}},
+            {"d1": {"d2": [1, 2, 3, 4, 5]}},
+            (),
+            id="no_conflict_list_merge",
+        ),
+        # The next example passes just fine, which is intentional, but the test is here
+        # to highlight the behaviour
+        pytest.param(
+            {"d1": {"d2": [1, 2, 3]}},
+            {"d1": {"d2": [{"file": "a_path"}]}},
+            {"d1": {"d2": [1, 2, 3, {"file": "a_path"}]}},
+            (),
+            id="no_conflict_list_merge_dubious_content",
+        ),
+        pytest.param(
+            {"d1": {"d2": [1, 2, 3]}},
+            {"d1": {"d2": "a"}},
+            {"d1": {"d2": "a"}},
+            ("d1.d2",),
+            id="conflict_list_and_not_list",
+        ),
     ],
 )
 def test_config_merge(dest, source, exp_result, exp_conflicts):

virtual_ecosystem/core/config.py

@@ -34,10 +34,22 @@ def config_merge(
     """Recursively merge two dictionaries detecting duplicated key definitions.
 
     This function returns a copy of the input ``dest`` dictionary that has been extended
-    recursively with the entries from the input ``source`` dictionary. The two input
-    dictionaries must not share any key paths and when duplicated key paths are
-    found, the value from the source dictionary is used and the function extends the
-    returned ``conflicts`` tuple with the duplicated key path.
+    recursively with the entries from the input ``source`` dictionary.
+
+    In general, if two input dictionaries share complete key paths (that is a set of
+    nested dictionary keys leading to a value) then that indicates a duplicated setting.
+    The values might be identical, but the configuration files should not duplicate
+    settings. When  duplicated key paths are found, the value from the source dictionary
+    is used and the function extends the returned ``conflicts`` tuple with the
+    duplicated key path.
+
+    However an exception is where both entries are lists - resulting from a TOML array
+    of tables (https://toml.io/en/v1.0.0#array-of-tables). In this case, it is
+    reasonable to append the source values to the destination values. The motivating
+    example here are `[[core.data.variable]]` entries, which can quite reasonably be
+    split across configuration sources. Note that no attempt is made to check that the
+    combined values are congruent - this is deferred to error handling when the
+    configuration is used.
 
     Args:
         dest: A dictionary to extend
@@ -66,6 +78,9 @@ def config_merge(
             dest[src_key], conflicts = config_merge(
                 dest_val, src_val, conflicts=conflicts, path=next_path
             )
+        elif isinstance(dest_val, list) and isinstance(src_val, list):
+            # Both values for this key are lists, so merge the lists
+            dest[src_key] = [*dest_val, *src_val]
         elif dest_val is None:
             # The key is not currently in dest, so add the key value pair
             dest[src_key] = src_val

virtual_ecosystem/core/data.py

@@ -107,11 +107,11 @@
     [[core.data.variable]]
     var_name="elev"
 
-Data configurations must not contain repeated data variable names. NOTE: At the moment,
-```core.data.variable``` tags cannot be used across multiple toml config files without
-causing ```ConfigurationError: Duplicated entries in config files: core.data.variable```
-to be raised. This means that all variables need to be combined in one ```config```
-file.
+Data configurations must not contain repeated data variable names.
+
+You can include ```core.data.variable``` tags in different files. This can be useful to
+group model-specific data with other model configuration options, and allow
+configuration files to be swapped in a more modular fashion.
 
 .. code-block:: python