Update history attribute

cf_xarray/tracking.py

@@ -4,6 +4,7 @@
 
 import copy
 import functools
+from datetime import datetime
 
 CELL_METHODS = {
     "sum": "sum",
@@ -25,7 +26,23 @@ def add_cell_methods(attrs, context):
 
 
 def add_history(attrs, context):
-    return {"history": None}
+    """Adds a history attribute following the NetCDF User Guide convention."""
+
+    # https://www.unidata.ucar.edu/software/netcdf/documentation/4.7.4-pre/attribute_conventions.html
+    # A global attribute for an audit trail. This is a character array with a line
+    # for each invocation of a program that has modified the dataset. Well-behaved
+    # generic netCDF applications should append a line containing:
+    #     date, time of day, user name, program name and command arguments.
+
+    # nco uses the ctime format
+    now = datetime.now().ctime()
+    history = attrs[0].get("history", [])
+    new_history = (
+        f"{now}:"
+        f" {context.func}(args)\n"
+        # TODO: should we record software versions?
+    )
+    return {"history": history + [new_history]}
 
 
 def _tracker(
@@ -63,6 +80,8 @@ def track_cf_attributes(
     history: bool
         Adds a history attribute like NCO and follows the NUG convention.
     """
+
+    # TODO: check xarray version here.
     return functools.partial(
         _tracker, strict=strict, cell_methods=cell_methods, history=history
     )