Implement cftime vectorization as discussed in PR #8322 (#8324)

* feat: implement as discussed in PR #8322

* fix: Stick closer to the original function and add PR suggestions

* Update xarray/coding/times.py

Co-authored-by: Michael Niklas  <mick.niklas@gmail.com>

* Update xarray/coding/times.py

* Update xarray/coding/times.py

* Add what's new entry

---------

Co-authored-by: Illviljan <14371165+Illviljan@users.noreply.github.com>
Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>
Co-authored-by: Michael Niklas <mick.niklas@gmail.com>
Co-authored-by: Kai Mühlbauer <kai.muehlbauer@uni-bonn.de>
Co-authored-by: Kai Mühlbauer <kmuehlbauer@wradlib.org>
Co-authored-by: Spencer Clark <spencerkclark@gmail.com>

Author: 6 people

doc/whats-new.rst

@@ -65,7 +65,8 @@ Performance
   in :py:class:`~xarray.indexing.VectorizedIndexer` and :py:class:`~xarray.indexing.OuterIndexer`
   (:issue:`10316`).
   By `Jesse Rusak <https://github.com/jder>`_.
-
+- Speed up encoding of :py:class:`cftime.datetime` objects by roughly a factor
+  of three (:pull:`8324`). By `Antoine Gibek <https://github.com/antscloud>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/coding/times.py

@@ -946,41 +946,38 @@ def _encode_datetime_with_cftime(dates, units: str, calendar: str) -> np.ndarray
     else:
         cftime = attempt_import("cftime")
 
+    dates = np.asarray(dates)
+    original_shape = dates.shape
+
     if np.issubdtype(dates.dtype, np.datetime64):
         # numpy's broken datetime conversion only works for us precision
         dates = dates.astype("M8[us]").astype(datetime)
 
-    def wrap_dt(dt):
-        # convert to cftime proleptic gregorian in case of datetime.datetime
-        # needed because of https://github.com/Unidata/cftime/issues/354
-        if isinstance(dt, datetime) and not isinstance(dt, cftime.datetime):
-            dt = cftime.datetime(
-                dt.year,
-                dt.month,
-                dt.day,
-                dt.hour,
-                dt.minute,
-                dt.second,
-                dt.microsecond,
-                calendar="proleptic_gregorian",
-            )
-        return dt
+    dates = np.atleast_1d(dates)
 
-    def encode_datetime(d):
-        # Since netCDF files do not support storing float128 values, we ensure
-        # that float64 values are used by setting longdouble=False in num2date.
-        # This try except logic can be removed when xarray's minimum version of
-        # cftime is at least 1.6.2.
-        try:
-            return (
-                np.nan
-                if d is None
-                else cftime.date2num(wrap_dt(d), units, calendar, longdouble=False)
-            )
-        except TypeError:
-            return np.nan if d is None else cftime.date2num(wrap_dt(d), units, calendar)
+    # Find all the None position
+    none_position = dates == None  # noqa: E711
+    filtered_dates = dates[~none_position]
+
+    # Since netCDF files do not support storing float128 values, we ensure
+    # that float64 values are used by setting longdouble=False in num2date.
+    # This try except logic can be removed when xarray's minimum version of
+    # cftime is at least 1.6.2.
+    try:
+        encoded_nums = cftime.date2num(
+            filtered_dates, units, calendar, longdouble=False
+        )
+    except TypeError:
+        encoded_nums = cftime.date2num(filtered_dates, units, calendar)
+
+    if filtered_dates.size == none_position.size:
+        return encoded_nums.reshape(original_shape)
 
-    return reshape(np.array([encode_datetime(d) for d in ravel(dates)]), dates.shape)
+    # Create a full matrix of NaN
+    # And fill the num dates in the not NaN or None position
+    result = np.full(dates.shape, np.nan)
+    result[np.nonzero(~none_position)] = encoded_nums
+    return result.reshape(original_shape)
 
 
 def cast_to_int_if_safe(num) -> np.ndarray: