Add doc

Co-authored-by: Copilot <copilot@github.com>

Author: b8raoult

docs/cli/extract.rst

@@ -0,0 +1,55 @@
+.. _extract_command:
+
+Extract Command
+===============
+
+The ``extract`` command extracts constant fields and/or climatologies from a
+dataset and writes them to a NetCDF file following CF conventions.
+
+This is useful for producing ancillary files needed during inference, such as
+orography (a constant) or monthly mean sea-surface temperature (a climatology).
+
+The ``DATASET`` argument can be:
+
+- A dataset name or path (resolved by ``open_dataset``).
+- A YAML or JSON file (ending in ``.yaml`` or ``.json``) whose content is
+  loaded and passed to ``open_dataset``.
+
+Usage examples
+--------------
+
+Extract a constant field:
+
+.. code:: console
+
+   $ anemoi-datasets extract --constant z dataset-name output.nc
+
+Extract multiple constants:
+
+.. code:: console
+
+   $ anemoi-datasets extract --constant z --constant lsm dataset-name output.nc
+
+Extract climatologies (one value per month):
+
+.. code:: console
+
+   $ anemoi-datasets extract --climatology sst --climatology ci dataset-name output.nc
+
+Mix constants and climatologies:
+
+.. code:: console
+
+   $ anemoi-datasets extract --constant z --climatology sst dataset-name output.nc
+
+Use a YAML file to specify the dataset:
+
+.. code:: console
+
+   $ anemoi-datasets extract --constant z dataset.yaml output.nc
+
+.. argparse::
+    :module: anemoi.datasets.__main__
+    :func: create_parser
+    :prog: anemoi-datasets
+    :path: extract

docs/cli/introduction.rst

@@ -40,3 +40,4 @@ The commands are:
    cli/patch
    cli/compare-lam
    cli/validate
+   cli/extract

src/anemoi/datasets/commands/extract.py

@@ -53,7 +53,20 @@ def run(self, args: Any) -> None:
 
         from anemoi.datasets import open_dataset
 
-        ds = open_dataset(args.dataset)
+        dataset = args.dataset
+
+        if dataset.endswith(".yaml") or dataset.endswith(".json"):
+            import json
+
+            import yaml
+
+            with open(dataset) as f:
+                if dataset.endswith(".yaml"):
+                    dataset = yaml.safe_load(f)
+                else:
+                    dataset = json.load(f)
+
+        ds = open_dataset(dataset)
 
         constants = args.constant
         climatologies = args.climatology