Type check sentinel values (#3472)

* type check sentinel values, using Enum pattern

* Code review

* Code review

* Code review

* Code review

xarray/core/dataarray.py

@@ -53,7 +53,7 @@
 from .formatting import format_item
 from .indexes import Indexes, default_indexes
 from .options import OPTIONS
-from .utils import ReprObject, _check_inplace, either_dict_or_kwargs
+from .utils import Default, ReprObject, _default, _check_inplace, either_dict_or_kwargs
 from .variable import (
     IndexVariable,
     Variable,
@@ -270,8 +270,6 @@ class DataArray(AbstractArray, DataWithCoords):
     _coarsen_cls = rolling.DataArrayCoarsen
     _resample_cls = resample.DataArrayResample
 
-    __default = ReprObject("<default>")
-
     dt = property(DatetimeAccessor)
 
     def __init__(
@@ -387,18 +385,18 @@ def _replace(
         self,
         variable: Variable = None,
         coords=None,
-        name: Optional[Hashable] = __default,
+        name: Union[Hashable, None, Default] = _default,
     ) -> "DataArray":
         if variable is None:
             variable = self.variable
         if coords is None:
             coords = self._coords
-        if name is self.__default:
+        if name is _default:
             name = self.name
         return type(self)(variable, coords, name=name, fastpath=True)
 
     def _replace_maybe_drop_dims(
-        self, variable: Variable, name: Optional[Hashable] = __default
+        self, variable: Variable, name: Union[Hashable, None, Default] = _default
     ) -> "DataArray":
         if variable.dims == self.dims and variable.shape == self.shape:
             coords = self._coords.copy()
@@ -438,7 +436,7 @@ def _to_temp_dataset(self) -> Dataset:
         return self._to_dataset_whole(name=_THIS_ARRAY, shallow_copy=False)
 
     def _from_temp_dataset(
-        self, dataset: Dataset, name: Hashable = __default
+        self, dataset: Dataset, name: Hashable = _default
     ) -> "DataArray":
         variable = dataset._variables.pop(_THIS_ARRAY)
         coords = dataset._variables
@@ -2450,13 +2448,11 @@ def identical(self, other: "DataArray") -> bool:
         except (TypeError, AttributeError):
             return False
 
-    __default_name = object()
-
     def _result_name(self, other: Any = None) -> Optional[Hashable]:
         # use the same naming heuristics as pandas:
         # https://github.com/ContinuumIO/blaze/issues/458#issuecomment-51936356
-        other_name = getattr(other, "name", self.__default_name)
-        if other_name is self.__default_name or other_name == self.name:
+        other_name = getattr(other, "name", _default)
+        if other_name is _default or other_name == self.name:
             return self.name
         else:
             return None

xarray/core/dataset.py

@@ -70,8 +70,10 @@
 from .options import OPTIONS, _get_keep_attrs
 from .pycompat import dask_array_type
 from .utils import (
+    Default,
     Frozen,
     SortedKeysDict,
+    _default,
     _check_inplace,
     decode_numpy_dict_values,
     either_dict_or_kwargs,
@@ -856,23 +858,18 @@ def _construct_direct(
         obj._accessors = None
         return obj
 
-    __default = object()
-
     @classmethod
     def _from_vars_and_coord_names(cls, variables, coord_names, attrs=None):
         return cls._construct_direct(variables, coord_names, attrs=attrs)
 
-    # TODO(shoyer): renable type checking on this signature when pytype has a
-    # good way to handle defaulting arguments to a sentinel value:
-    # https://github.com/python/mypy/issues/1803
-    def _replace(  # type: ignore
+    def _replace(
         self,
         variables: Dict[Hashable, Variable] = None,
         coord_names: Set[Hashable] = None,
         dims: Dict[Any, int] = None,
-        attrs: Optional[Dict[Hashable, Any]] = __default,
-        indexes: Optional[Dict[Any, pd.Index]] = __default,
-        encoding: Optional[dict] = __default,
+        attrs: Union[Dict[Hashable, Any], None, Default] = _default,
+        indexes: Union[Dict[Any, pd.Index], None, Default] = _default,
+        encoding: Union[dict, None, Default] = _default,
         inplace: bool = False,
     ) -> "Dataset":
         """Fastpath constructor for internal use.
@@ -890,12 +887,12 @@ def _replace(  # type: ignore
                 self._coord_names = coord_names
             if dims is not None:
                 self._dims = dims
-            if attrs is not self.__default:
-                self._attrs = attrs
-            if indexes is not self.__default:
-                self._indexes = indexes
-            if encoding is not self.__default:
-                self._encoding = encoding
+            if attrs is not _default:
+                self._attrs = attrs  # type: ignore # FIXME need mypy 0.750
+            if indexes is not _default:
+                self._indexes = indexes  # type: ignore # FIXME need mypy 0.750
+            if encoding is not _default:
+                self._encoding = encoding  # type: ignore # FIXME need mypy 0.750
             obj = self
         else:
             if variables is None:
@@ -904,23 +901,23 @@ def _replace(  # type: ignore
                 coord_names = self._coord_names.copy()
             if dims is None:
                 dims = self._dims.copy()
-            if attrs is self.__default:
+            if attrs is _default:
                 attrs = copy.copy(self._attrs)
-            if indexes is self.__default:
+            if indexes is _default:
                 indexes = copy.copy(self._indexes)
-            if encoding is self.__default:
+            if encoding is _default:
                 encoding = copy.copy(self._encoding)
             obj = self._construct_direct(
                 variables, coord_names, dims, attrs, indexes, encoding
             )
         return obj
 
-    def _replace_with_new_dims(  # type: ignore
+    def _replace_with_new_dims(
         self,
         variables: Dict[Hashable, Variable],
         coord_names: set = None,
-        attrs: Optional[Dict[Hashable, Any]] = __default,
-        indexes: Dict[Hashable, pd.Index] = __default,
+        attrs: Union[Dict[Hashable, Any], None, Default] = _default,
+        indexes: Union[Dict[Hashable, pd.Index], None, Default] = _default,
         inplace: bool = False,
     ) -> "Dataset":
         """Replace variables with recalculated dimensions."""
@@ -929,12 +926,12 @@ def _replace_with_new_dims(  # type: ignore
             variables, coord_names, dims, attrs, indexes, inplace=inplace
         )
 
-    def _replace_vars_and_dims(  # type: ignore
+    def _replace_vars_and_dims(
         self,
         variables: Dict[Hashable, Variable],
         coord_names: set = None,
         dims: Dict[Hashable, int] = None,
-        attrs: Dict[Hashable, Any] = __default,
+        attrs: Union[Dict[Hashable, Any], None, Default] = _default,
         inplace: bool = False,
     ) -> "Dataset":
         """Deprecated version of _replace_with_new_dims().

xarray/core/utils.py

@@ -6,6 +6,7 @@
 import os.path
 import re
 import warnings
+from enum import Enum
 from typing import (
     AbstractSet,
     Any,
@@ -701,3 +702,11 @@ def get_temp_dimname(dims: Container[Hashable], new_dim: Hashable) -> Hashable:
     while new_dim in dims:
         new_dim = "_" + str(new_dim)
     return new_dim
+
+
+# Singleton type, as per https://github.com/python/typing/pull/240
+class Default(Enum):
+    token = 0
+
+
+_default = Default.token