Declare soma_type as a class var rather than a property. (#102)

When using `@property` declarations, the type system would not realize
that in the implementation, `soma_type` could always be accessed as a
class variable. This change just makes `soma_type` a class var and
shuffles around some annotations to make that easier. The `is_sparse`
field on `NDArray` is similarly updated.

python-spec/src/somacore/base.py

@@ -5,7 +5,7 @@
 """
 
 import abc
-from typing import Any, MutableMapping, Optional, Type, TypeVar
+from typing import Any, ClassVar, MutableMapping, Optional, Type, TypeVar
 
 from typing_extensions import LiteralString
 
@@ -63,11 +63,14 @@ def metadata(self) -> MutableMapping[str, Any]:
         """
         raise NotImplementedError()
 
-    @property
-    @abc.abstractmethod
-    def soma_type(self) -> LiteralString:
-        """A string describing the SOMA type of this object."""
-        raise NotImplementedError()
+    soma_type: ClassVar[LiteralString]
+    """A string describing the SOMA type of this object. This is constant."""
+    # This uses ClassVar since you can't do abstract class properties.
+    # This is the equivalent, just without abc-based automatic verification.
+    #
+    # Overrides are marked Final with an ignore[misc] because mypy by default
+    # wants this to be mutable, and doesn't like overriding the mutable member
+    # with a Final member.
 
     # Context management
 

python-spec/src/somacore/collection.py

@@ -12,7 +12,6 @@
 )
 
 import pyarrow as pa
-from typing_extensions import LiteralString
 
 from . import base
 from . import data
@@ -33,6 +32,7 @@ class Collection(base.SOMAObject, MutableMapping[str, _ST], metaclass=abc.ABCMet
     """
 
     __slots__ = ()
+    soma_type = "SOMACollection"
 
     @classmethod
     @abc.abstractmethod
@@ -182,12 +182,6 @@ def set(
         """
         raise NotImplementedError()
 
-    # This is implemented as a property and not a literal so that it can be
-    # overridden with `Final` members in Collection specializations.
-    @property
-    def soma_type(self) -> LiteralString:
-        return "SOMACollection"
-
 
 class SimpleCollection(Collection[_ST]):
     """A memory-backed SOMA Collection for ad-hoc collection building.

python-spec/src/somacore/data.py

@@ -7,10 +7,20 @@
 """
 
 import abc
-from typing import Any, Iterator, Optional, Sequence, Tuple, Type, TypeVar, Union
+from typing import (
+    Any,
+    ClassVar,
+    Iterator,
+    Optional,
+    Sequence,
+    Tuple,
+    Type,
+    TypeVar,
+    Union,
+)
 
 import pyarrow as pa
-from typing_extensions import Final
+from typing_extensions import Final, Literal
 
 from . import base
 from . import options
@@ -25,6 +35,7 @@ class DataFrame(base.SOMAObject, metaclass=abc.ABCMeta):
     """A multi-column table with a user-defined schema."""
 
     __slots__ = ()
+    soma_type: Final = "SOMADataFrame"  # type: ignore[misc]
 
     # Lifecycle
 
@@ -89,10 +100,6 @@ def index_column_names(self) -> Tuple[str, ...]:
         """The names of the index (dimension) columns."""
         raise NotImplementedError()
 
-    # Basic operations
-
-    soma_type: Final = "SOMADataFrame"
-
 
 _NDT = TypeVar("_NDT", bound="NDArray")
 """Any implementation of NDArray."""
@@ -138,17 +145,16 @@ def schema(self) -> pa.Schema:
         """The schema of the data in this array."""
         raise NotImplementedError()
 
-    @property
-    @abc.abstractmethod
-    def is_sparse(self) -> bool:
-        """True if this array is sparse. False if this array is dense."""
-        raise NotImplementedError()
+    is_sparse: ClassVar[Literal[True, False]]
+    """True if the array is sparse. False if it is dense."""
 
 
 class DenseNDArray(NDArray, metaclass=abc.ABCMeta):
     """A N-dimensional array stored densely."""
 
     __slots__ = ()
+    soma_type: Final = "SOMADenseNDArray"  # type: ignore[misc]
+    is_sparse: Final = False  # type: ignore[misc]
 
     @abc.abstractmethod
     def read(
@@ -180,9 +186,6 @@ def write(
         """
         raise NotImplementedError()
 
-    is_sparse: Final = False
-    soma_type: Final = "SOMADenseNDArray"
-
 
 SparseArrowData = Union[
     pa.SparseCSCMatrix,
@@ -197,6 +200,8 @@ class SparseNDArray(NDArray, metaclass=abc.ABCMeta):
     """A N-dimensional array stored sparsely."""
 
     __slots__ = ()
+    soma_type: Final = "SparseNDArray"  # type: ignore[misc]
+    is_sparse: Final = True  # type: ignore[misc]
 
     @abc.abstractmethod
     def read(
@@ -242,9 +247,6 @@ def nnz(self) -> int:
         """
         raise NotImplementedError()
 
-    is_sparse: Final = True
-    soma_type: Final = "SOMASparseNDArray"
-
 
 #
 # Read types

python-spec/src/somacore/experiment.py

@@ -29,6 +29,7 @@ class Experiment(MutableMapping[str, _ST]):
     #     TypeError: multiple bases have instance lay-out conflict
 
     __slots__ = ()
+    soma_type: Final = "SOMAExperiment"
 
     obs = _mixin.item(data.DataFrame)
     """Primary observations on the observation axis.
@@ -63,8 +64,6 @@ def axis_query(
             var_query=var_query or query.AxisQuery(),
         )
 
-    soma_type: Final = "SOMAExperiment"
-
 
-class SimpleExperiment(Experiment, collection.SimpleCollection):
+class SimpleExperiment(Experiment, collection.SimpleCollection):  # type: ignore[misc]
     """An in-memory Collection with Experiment semantics."""

python-spec/src/somacore/measurement.py

@@ -28,6 +28,7 @@ class Measurement(MutableMapping[str, _ST]):
     #     TypeError: multiple bases have instance lay-out conflict
 
     __slots__ = ()
+    soma_type: Final = "SOMAMeasurement"
 
     var = _mixin.item(data.DataFrame)
     """Primary annotations on the variable axis for vars on this meansurement.
@@ -68,8 +69,6 @@ class Measurement(MutableMapping[str, _ST]):
     This is indexed by ``[varid_1, varid_2]``.
     """
 
-    soma_type: Final = "SOMAMeasurement"
-
 
-class SimpleMeasurement(Measurement, collection.SimpleCollection):
+class SimpleMeasurement(Measurement, collection.SimpleCollection):  # type: ignore[misc]
     """An in-memory Collection with Measurement semantics."""