WIP: Warn on datetime decoding error

xarray/coding/times.py

@@ -1370,15 +1370,24 @@ class CFDatetimeCoder(VariableCoder):
         May not be supported by all the backends.
     time_unit : PDDatetimeUnitOptions
           Target resolution when decoding dates. Defaults to "ns".
+    on_error  :  str, optional
+        What to do if there is an error when attempting to decode
+        a time variable. Options are: "raise", "warn", "ignore".
+        Defaults to "raise".
     """
 
     def __init__(
         self,
         use_cftime: bool | None = None,
         time_unit: PDDatetimeUnitOptions = "ns",
+        on_error: str = "raise",
     ) -> None:
         self.use_cftime = use_cftime
         self.time_unit = time_unit
+        if on_error in {"raise", "warn", "ignore"}:
+            self.on_error = on_error
+        else:
+            raise ValueError('on_error must be one of "raise", "warn", "ignore")')
 
     def encode(self, variable: Variable, name: T_Name = None) -> Variable:
         if np.issubdtype(variable.dtype, np.datetime64) or contains_cftime_datetimes(
@@ -1411,9 +1420,19 @@ def decode(self, variable: Variable, name: T_Name = None) -> Variable:
 
             units = pop_to(attrs, encoding, "units")
             calendar = pop_to(attrs, encoding, "calendar")
-            dtype = _decode_cf_datetime_dtype(
-                data, units, calendar, self.use_cftime, self.time_unit
-            )
+            try:
+                dtype = _decode_cf_datetime_dtype(
+                    data, units, calendar, self.use_cftime, self.time_unit
+                )
+            except ValueError as err:
+                if self.on_error == "ignore":
+                    return variable
+                elif self.on_error == "warn":
+                    emit_user_level_warning(err.args[0])
+                    return variable
+                else:
+                    raise
+
             transform = partial(
                 decode_cf_datetime,
                 units=units,

xarray/conventions.py

@@ -225,7 +225,22 @@ def decode_cf_variable(
                     "    ds = xr.open_dataset(decode_times=time_coder)\n",
                     FutureWarning,
                 )
-            decode_times = CFDatetimeCoder(use_cftime=use_cftime)
+            #            decode_times = CFDatetimeCoder(use_cftime=use_cftime)
+            decode_times_options = {
+                True: "raise",
+                "error": "raise",
+                "ignore": "ignore",
+                "warn": "warn",
+            }
+            try:
+                on_error = decode_times_options[decode_times]
+            except KeyError:
+                raise ValueError(
+                    "`decode_times` must be one of:"
+                    "True, False, 'raise', 'warn', 'ignore'"
+                ) from None
+            decode_times = CFDatetimeCoder(use_cftime=use_cftime, on_error=on_error)
+
         elif use_cftime is not None:
             raise TypeError(
                 "Usage of 'use_cftime' as a kwarg is not allowed "

xarray/tests/test_coding_times.py

@@ -2229,3 +2229,57 @@ def test_roundtrip_empty_datetime64_array(time_unit: PDDatetimeUnitOptions) -> N
     )
     assert_identical(variable, roundtripped)
     assert roundtripped.dtype == variable.dtype
+
+
+@requires_cftime
+def test_on_error_raises():
+    """
+    By default, decoding errors should raise
+    """
+    array = np.array([0, 1, 2], dtype=np.dtype("int64"))
+    encoded = Variable(["time"], array, attrs={"units": "ms since 00:00:00"})
+
+    # default is "raise"
+    coder = CFDatetimeCoder()
+
+    with pytest.raises(ValueError):
+        coder.decode(encoded)
+
+    # setting to "raise" should do the same thing.
+    coder = CFDatetimeCoder(on_error="raise")
+
+    with pytest.raises(ValueError):
+        coder.decode(encoded)
+
+
+@requires_cftime
+def test_on_error_ignore():
+    """
+    If on_error="ignore", no change.
+    """
+    array = np.array([0, 1, 2], dtype=np.dtype("int64"))
+    encoded = Variable(["time"], array, attrs={"units": "ms since 00:00:00"})
+
+    coder = CFDatetimeCoder(on_error="ignore")
+
+    decoded = coder.decode(encoded)
+
+    # it shouldn't have changed the variable
+    assert decoded is encoded
+
+
+@requires_cftime
+def test_on_error_warn():
+    """
+    If on_error="warn", no change, with a warning.
+    """
+    array = np.array([0, 1, 2], dtype=np.dtype("int64"))
+    encoded = Variable(["time"], array, attrs={"units": "ms since 00:00:00"})
+
+    coder = CFDatetimeCoder(on_error="warn")
+
+    with pytest.warns(UserWarning, match="unable to decode time units"):
+        decoded = coder.decode(encoded)
+
+    # it shouldn't have changed the variable
+    assert decoded is encoded