From e4c50143523508013ab20e5d219e9bd8491998db Mon Sep 17 00:00:00 2001
From: dcherian <deepak@cherian.net>
Date: Tue, 3 Aug 2021 10:48:54 -0600
Subject: [PATCH 1/2] Add attribute tracking functionality

---
 cf_xarray/__init__.py |  2 ++
 cf_xarray/tracking.py | 68 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+)
 create mode 100644 cf_xarray/tracking.py

diff --git a/cf_xarray/__init__.py b/cf_xarray/__init__.py
index f80585e6..d9afa399 100644
--- a/cf_xarray/__init__.py
+++ b/cf_xarray/__init__.py
@@ -1,6 +1,8 @@
+from . import tracking  # noqa
 from .accessor import CFAccessor  # noqa
 from .helpers import bounds_to_vertices, vertices_to_bounds  # noqa
 from .options import set_options  # noqa
+from .tracking import track_cf_attributes  # noqa
 from .utils import _get_version
 
 __version__ = _get_version()
diff --git a/cf_xarray/tracking.py b/cf_xarray/tracking.py
new file mode 100644
index 00000000..8d11e08a
--- /dev/null
+++ b/cf_xarray/tracking.py
@@ -0,0 +1,68 @@
+# This module provides functions for adding CF attribtues
+# and tracking history, provenance using xarray's keep_attrs
+# functionality
+
+import copy
+import functools
+
+CELL_METHODS = {
+    "sum": "sum",
+    "max": "maximum",
+    "min": "minimum",
+    "median": "median",
+    "mean": "mean",
+    "std": "standard_deviation",
+    "var": "variance",
+}
+
+
+def add_cell_methods(attrs, context):
+    """Add appropriate cell_methods attribute."""
+    assert len(attrs) == 1
+    cell_methods = attrs[0].get("cell_methods", "")
+    # TODO: get dim_name from context
+    return {"cell_methods": f"dim_name: {CELL_METHODS[context.func]} {cell_methods}"}
+
+
+def add_history(attrs, context):
+    return {"history": None}
+
+
+def _tracker(
+    attrs,
+    context,
+    strict: bool = False,
+    cell_methods: bool = True,
+    history: bool = True,
+):
+
+    # can only handle single variable attrs for now
+    assert len(attrs) == 1
+    attrs_out = copy.deepcopy(attrs[0])
+
+    if cell_methods and context.func in CELL_METHODS:
+        attrs_out.update(add_cell_methods(attrs, context))
+    if history:
+        attrs_out.update(add_history(attrs, context))
+        pass
+    return attrs_out
+
+
+def track_cf_attributes(
+    *, strict: bool = False, cell_methods: bool = True, history: bool = True
+):
+    """Top-level user-facing function.
+
+    Parameters
+    ----------
+    strict: bool
+        Controls if an error is raised when an appropriate attribute cannot
+        be added because of lack of information.
+    cell_methods: bool
+        Add cell_methods attribute when possible
+    history: bool
+        Adds a history attribute like NCO and follows the NUG convention.
+    """
+    return functools.partial(
+        _tracker, strict=strict, cell_methods=cell_methods, history=history
+    )

From e972ddedebe6f09b33e504c8c2ce7b9c458f7e9f Mon Sep 17 00:00:00 2001
From: dcherian <deepak@cherian.net>
Date: Tue, 3 Aug 2021 16:50:38 -0600
Subject: [PATCH 2/2] Update history attribute

---
 cf_xarray/tracking.py | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/cf_xarray/tracking.py b/cf_xarray/tracking.py
index 8d11e08a..e559774a 100644
--- a/cf_xarray/tracking.py
+++ b/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
     )