✨ feat: add Quantity API

pyproject.toml

@@ -141,6 +141,7 @@ ignore = [
   "PLR09",    # Too many <...>
   "PLR2004",  # Magic value used in comparison
   "RET505",   # Unnecessary `else`/`elif` after `return` statement
+  "RUF022",   # `__all__` is not sorted
 ]
 
 [tool.ruff.lint.per-file-ignores]

src/quantity/__init__.py

@@ -3,7 +3,13 @@
 Copyright (c) 2024 Astropy Developers. All rights reserved.
 """
 
+from . import api
 from ._src import Quantity
 from .version import version as __version__  # noqa: F401
 
-__all__ = ["Quantity"]
+__all__ = [
+    # modules
+    "api",
+    # functions and classes
+    "Quantity",
+]

src/quantity/_src/api.py

@@ -0,0 +1,96 @@
+"""The Quantity API. Private module."""
+
+__all__ = ["Quantity", "QuantityArray", "Unit"]
+
+from typing import Protocol, runtime_checkable
+
+from astropy.units import UnitBase as Unit
+
+from .array_api import Array
+
+
+@runtime_checkable
+class Quantity(Protocol):
+    """Minimal definition of the Quantity API.
+
+    At minimum a Quantity must have the following attributes:
+
+    - `value`: the numerical value of the quantity (adhering to the Array API)
+    - `unit`: the unit of the quantity
+
+    In practice, Quantities themselves must adhere to the Array API, not just
+    their values. This stricter requirement is described by the `QuantityArray`
+    protocol.
+
+    See Also
+    --------
+    QuantityArray : A Quantity that adheres to the Array API
+
+    Examples
+    --------
+    >>> import numpy as np
+    >>> import astropy.units as u
+    >>> from quantity import Quantity
+    >>> from quantity import api
+
+    >>> issubclass(Quantity, api.Quantity)
+    True
+
+    >>> q = Quantity(value=np.array([1, 2, 3]), unit=u.m)
+    >>> isinstance(q, api.Quantity)
+    True
+
+    """
+
+    #: The numerical value of the quantity, adhering to the Array API.
+    value: Array
+
+    #: The unit of the quantity.
+    unit: Unit
+
+    @classmethod
+    def __subclasshook__(cls: type, c: type) -> bool:
+        """Enable the subclass check for data descriptors."""
+        return (
+            hasattr(c, "value") or "value" in getattr(c, "__annotations__", ())
+        ) and (hasattr(c, "unit") or "unit" in getattr(c, "__annotations__", ()))
+
+
+@runtime_checkable
+class QuantityArray(Quantity, Array, Protocol):
+    """An array-valued Quantity.
+
+    A QuantityArray is a Quantity that itself adheres to the Array API. This
+    means that the QuantityArray has properties like `shape`, `dtype`, and the
+    `__array_namespace__` method, among many other properties and methods. To
+    understand the full requirements of the Array API, see the `Array` protocol.
+    The `Quantity` protocol describes the minimal requirements for a Quantity,
+    separate from the Array API. QuantityArray is the combination of these two
+    protocols and is the most complete description of a Quantity.
+
+    See Also
+    --------
+    Quantity : The minimal Quantity API, separate from the Array API
+
+    Examples
+    --------
+    >>> import numpy as np
+    >>> import astropy.units as u
+    >>> from quantity import Quantity
+    >>> from quantity import api
+
+    >>> issubclass(Quantity, api.QuantityArray)
+    True
+
+    >>> q = Quantity(value=np.array([1, 2, 3]), unit=u.m)
+    >>> isinstance(q, api.QuantityArray)
+    True
+
+    """
+
+    ...
+
+    @classmethod
+    def __subclasshook__(cls: type, c: type) -> bool:
+        """Enable the subclass check for data descriptors."""
+        return Quantity.__subclasscheck__(c) and issubclass(c, Array)

src/quantity/_src/array_api.py

@@ -0,0 +1,25 @@
+"""Minimal definition of the Array API.
+
+NOTE: this module will be deprecated when
+https://github.com/data-apis/array-api-typing is released.
+
+"""
+
+from __future__ import annotations
+
+__all__ = ["HasArrayNameSpace", "Array"]
+
+from typing import Any, Protocol, runtime_checkable
+
+
+class HasArrayNameSpace(Protocol):
+    """Minimal definition of the Array API."""
+
+    def __array_namespace__(self) -> Any: ...
+
+
+@runtime_checkable
+class Array(HasArrayNameSpace, Protocol):
+    """Minimal definition of the Array API."""
+
+    def __pow__(self, other: Any) -> Array: ...

src/quantity/_src/core.py

@@ -10,25 +10,27 @@
 import numpy as np
 from astropy.units.quantity_helper import UFUNC_HELPERS
 
+from .api import QuantityArray
 from .utils import has_array_namespace
 
 if TYPE_CHECKING:
     from typing import Any
 
+    from .api import Unit
+    from .array_api import Array
+
 
 DIMENSIONLESS = u.dimensionless_unscaled
 
 PYTHON_NUMBER = float | int | complex
 
 
-def get_value_and_unit(arg, default_unit=None):
-    # HACK: interoperability with astropy Quantity.  Have protocol?
-    try:
-        unit = arg.unit
-    except AttributeError:
-        return arg, default_unit
-    else:
-        return arg.value, unit
+def get_value_and_unit(
+    arg: QuantityArray | Array, default_unit: Unit | None = None
+) -> tuple[Array, Unit]:
+    return (
+        (arg.value, arg.unit) if isinstance(arg, QuantityArray) else (arg, default_unit)
+    )
 
 
 def value_in_unit(value, unit):

src/quantity/api.py

@@ -0,0 +1,16 @@
+"""Quantity-2.0: the Quantity API.
+
+This module provides runtime-checkable Protocol objects that define the Quantity
+API. In particular there are:
+
+- `Quantity`: the minimal definition of a Quantity, separate from the Array API.
+- `QuantityArray`: a Quantity that adheres to the Array API. This is the most
+    complete definition of a Quantity, inheriting from the Quantity API and
+    adding the requirements for the Array API.
+
+"""
+# Copyright (c) 2024 Astropy Developers. All rights reserved.
+
+from ._src.api import Quantity, QuantityArray
+
+__all__ = ["Quantity", "QuantityArray"]

tests/test_api.py

@@ -0,0 +1,58 @@
+# Licensed under a 3-clause BSD style license - see LICENSE.rst
+"""Test the Quantity class Array API compatibility."""
+
+import astropy.units as u
+import numpy as np
+import pytest
+
+from quantity import Quantity, api
+
+from .conftest import ARRAY_NAMESPACES
+
+
+def test_issubclass_api():
+    """Test that Quantity is a subclass of api.Quantity and api.QuantityArray."""
+    assert issubclass(Quantity, api.Quantity)
+    assert issubclass(Quantity, api.QuantityArray)
+
+
+def test_ndarray():
+    """Test that ndarray does not satisfy the Quantity API."""
+    assert not issubclass(np.ndarray, api.Quantity)
+    assert not isinstance(np.array([1, 2, 3]), api.Quantity)
+
+
+def test_astropy_quantity():
+    """Test that astropy.units.Quantity works with the Quantity API."""
+    assert issubclass(u.Quantity, api.Quantity)
+    assert isinstance(u.Quantity(np.array([1, 2, 3]), u.m), api.Quantity)
+
+
+# ------------------------------
+
+
+@pytest.fixture
+def array_and_quantity(request):
+    xp = request.param.xp
+    value = xp.asarray([1.0, 2.0, 3.0])
+    q = Quantity(value, u.m)
+    return value, q
+
+
+@pytest.mark.parametrize("array_and_quantity", ARRAY_NAMESPACES, indirect=True)
+class TestIsinstanceAPI:
+    """Check Quantities are properly recognized independent of the array type."""
+
+    def test_issubclass_api(self, array_and_quantity):
+        v, q = array_and_quantity
+        assert not issubclass(type(v), api.Quantity)
+        assert not issubclass(type(v), api.QuantityArray)
+        assert issubclass(type(q), api.Quantity)
+        assert issubclass(type(q), api.QuantityArray)
+
+    def test_isinstance_api(self, array_and_quantity):
+        v, q = array_and_quantity
+        assert not isinstance(v, api.Quantity)
+        assert not isinstance(v, api.QuantityArray)
+        assert isinstance(q, api.Quantity)
+        assert isinstance(q, api.QuantityArray)