Per-variable specification of boolean parameters in open_dataset (#9218)

* allow per-variable choice of mask_and_scale in open_dataset

* simplify docstring datatype

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* dict -> Mapping in type annotation

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

* use typevar for _item_or_default annotation

Otherwise you lose all typing when you use that because it returns Any.

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* implement feature for 4 additional parameters

* fix default value inconsistency

* add what's new + None annotation

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* _item_or_default return type T | None

* remove deault default value _item_or_default

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* docstring dtype naming

---------

Co-authored-by: Mathijs Verhaegh <mathijs@chromodynamics.nl>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Michael Niklas <mick.niklas@gmail.com>

doc/whats-new.rst

@@ -22,6 +22,9 @@ v2024.06.1 (unreleased)
 
 New Features
 ~~~~~~~~~~~~
+- Allow per-variable specification of ``mask_and_scale``, ``decode_times``, ``decode_timedelta``
+  ``use_cftime`` and ``concat_characters`` params in :py:func:`~xarray.open_dataset`  (:pull:`9218`).
+  By `Mathijs Verhaegh <https://github.com/Ostheer>`_.
 - Allow chunking for arrays with duplicated dimension names (:issue:`8759`, :pull:`9099`).
   By `Martin Raspaud <https://github.com/mraspaud>`_.
 - Extract the source url from fsspec objects (:issue:`9142`, :pull:`8923`).

xarray/backends/api.py

@@ -398,11 +398,11 @@ def open_dataset(
     chunks: T_Chunks = None,
     cache: bool | None = None,
     decode_cf: bool | None = None,
-    mask_and_scale: bool | None = None,
-    decode_times: bool | None = None,
-    decode_timedelta: bool | None = None,
-    use_cftime: bool | None = None,
-    concat_characters: bool | None = None,
+    mask_and_scale: bool | Mapping[str, bool] | None = None,
+    decode_times: bool | Mapping[str, bool] | None = None,
+    decode_timedelta: bool | Mapping[str, bool] | None = None,
+    use_cftime: bool | Mapping[str, bool] | None = None,
+    concat_characters: bool | Mapping[str, bool] | None = None,
     decode_coords: Literal["coordinates", "all"] | bool | None = None,
     drop_variables: str | Iterable[str] | None = None,
     inline_array: bool = False,
@@ -451,25 +451,31 @@ def open_dataset(
     decode_cf : bool, optional
         Whether to decode these variables, assuming they were saved according
         to CF conventions.
-    mask_and_scale : bool, optional
+    mask_and_scale : bool or dict-like, optional
         If True, replace array values equal to `_FillValue` with NA and scale
         values according to the formula `original_values * scale_factor +
         add_offset`, where `_FillValue`, `scale_factor` and `add_offset` are
         taken from variable attributes (if they exist).  If the `_FillValue` or
         `missing_value` attribute contains multiple values a warning will be
         issued and all array values matching one of the multiple values will
-        be replaced by NA. This keyword may not be supported by all the backends.
-    decode_times : bool, optional
+        be replaced by NA. Pass a mapping, e.g. ``{"my_variable": False}``,
+        to toggle this feature per-variable individually.
+        This keyword may not be supported by all the backends.
+    decode_times : bool or dict-like, optional
         If True, decode times encoded in the standard NetCDF datetime format
         into datetime objects. Otherwise, leave them encoded as numbers.
+        Pass a mapping, e.g. ``{"my_variable": False}``,
+        to toggle this feature per-variable individually.
         This keyword may not be supported by all the backends.
-    decode_timedelta : bool, optional
+    decode_timedelta : bool or dict-like, optional
         If True, decode variables and coordinates with time units in
         {"days", "hours", "minutes", "seconds", "milliseconds", "microseconds"}
         into timedelta objects. If False, leave them encoded as numbers.
         If None (default), assume the same value of decode_time.
+        Pass a mapping, e.g. ``{"my_variable": False}``,
+        to toggle this feature per-variable individually.
         This keyword may not be supported by all the backends.
-    use_cftime: bool, optional
+    use_cftime: bool or dict-like, optional
         Only relevant if encoded dates come from a standard calendar
         (e.g. "gregorian", "proleptic_gregorian", "standard", or not
         specified).  If None (default), attempt to decode times to
@@ -478,12 +484,16 @@ def open_dataset(
         ``cftime.datetime`` objects, regardless of whether or not they can be
         represented using ``np.datetime64[ns]`` objects.  If False, always
         decode times to ``np.datetime64[ns]`` objects; if this is not possible
-        raise an error. This keyword may not be supported by all the backends.
-    concat_characters : bool, optional
+        raise an error. Pass a mapping, e.g. ``{"my_variable": False}``,
+        to toggle this feature per-variable individually.
+        This keyword may not be supported by all the backends.
+    concat_characters : bool or dict-like, optional
         If True, concatenate along the last dimension of character arrays to
         form string arrays. Dimensions will only be concatenated over (and
         removed) if they have no corresponding variable and if they are only
         used as the last dimension of character arrays.
+        Pass a mapping, e.g. ``{"my_variable": False}``,
+        to toggle this feature per-variable individually.
         This keyword may not be supported by all the backends.
     decode_coords : bool or {"coordinates", "all"}, optional
         Controls which variables are set as coordinate variables:

xarray/conventions.py

@@ -2,7 +2,7 @@
 
 from collections import defaultdict
 from collections.abc import Hashable, Iterable, Mapping, MutableMapping
-from typing import TYPE_CHECKING, Any, Literal, Union
+from typing import TYPE_CHECKING, Any, Literal, TypeVar, Union
 
 import numpy as np
 import pandas as pd
@@ -384,16 +384,26 @@ def _update_bounds_encoding(variables: T_Variables) -> None:
                     bounds_encoding.setdefault("calendar", encoding["calendar"])
 
 
+T = TypeVar("T")
+
+
+def _item_or_default(obj: Mapping[Any, T] | T, key: Hashable, default: T) -> T:
+    """
+    Return item by key if obj is mapping and key is present, else return default value.
+    """
+    return obj.get(key, default) if isinstance(obj, Mapping) else obj
+
+
 def decode_cf_variables(
     variables: T_Variables,
     attributes: T_Attrs,
-    concat_characters: bool = True,
-    mask_and_scale: bool = True,
-    decode_times: bool = True,
+    concat_characters: bool | Mapping[str, bool] = True,
+    mask_and_scale: bool | Mapping[str, bool] = True,
+    decode_times: bool | Mapping[str, bool] = True,
     decode_coords: bool | Literal["coordinates", "all"] = True,
     drop_variables: T_DropVariables = None,
-    use_cftime: bool | None = None,
-    decode_timedelta: bool | None = None,
+    use_cftime: bool | Mapping[str, bool] | None = None,
+    decode_timedelta: bool | Mapping[str, bool] | None = None,
 ) -> tuple[T_Variables, T_Attrs, set[Hashable]]:
     """
     Decode several CF encoded variables.
@@ -431,7 +441,7 @@ def stackable(dim: Hashable) -> bool:
         if k in drop_variables:
             continue
         stack_char_dim = (
-            concat_characters
+            _item_or_default(concat_characters, k, True)
             and v.dtype == "S1"
             and v.ndim > 0
             and stackable(v.dims[-1])
@@ -440,12 +450,12 @@ def stackable(dim: Hashable) -> bool:
             new_vars[k] = decode_cf_variable(
                 k,
                 v,
-                concat_characters=concat_characters,
-                mask_and_scale=mask_and_scale,
-                decode_times=decode_times,
+                concat_characters=_item_or_default(concat_characters, k, True),
+                mask_and_scale=_item_or_default(mask_and_scale, k, True),
+                decode_times=_item_or_default(decode_times, k, True),
                 stack_char_dim=stack_char_dim,
-                use_cftime=use_cftime,
-                decode_timedelta=decode_timedelta,
+                use_cftime=_item_or_default(use_cftime, k, None),
+                decode_timedelta=_item_or_default(decode_timedelta, k, None),
             )
         except Exception as e:
             raise type(e)(f"Failed to decode variable {k!r}: {e}") from e