feat: astype: accept a kind of data type

Author: lucascolley

src/array_api_stubs/_draft/data_type_functions.py

@@ -16,7 +16,7 @@ def astype(
     x: array, dtype: dtype, /, *, copy: bool = True, device: Optional[device] = None
 ) -> array:
     """
-    Copies an array to a specified data type irrespective of :ref:`type-promotion` rules.
+    Copies an array to a specified data type irrespective of :ref:`type-promotion` rules, or to a *kind* of data type.
 
     .. note::
        Casting floating-point ``NaN`` and ``infinity`` values to integral data types is not specified and is implementation-dependent.
@@ -40,8 +40,14 @@ def astype(
     ----------
     x: array
         array to cast.
-    dtype: dtype
-        desired data type.
+    dtype_or_kind: Union[dtype, str]
+        desired data type or kind of data type. Supported kinds are:
+        -   ``'bool'``: boolean data types (e.g., ``bool``).
+        -   ``'signed integer'``: signed integer data types (e.g., ``int8``, ``int16``, ``int32``, ``int64``).
+        -   ``'unsigned integer'``: unsigned integer data types (e.g., ``uint8``, ``uint16``, ``uint32``, ``uint64``).
+        -   ``'integral'``: integer data types. Shorthand for ``('signed integer', 'unsigned integer')``.
+        -   ``'real floating'``: real-valued floating-point data types (e.g., ``float32``, ``float64``).
+        -   ``'complex floating'``: complex floating-point data types (e.g., ``complex64``, ``complex128``).
     copy: bool
         specifies whether to copy an array when the specified ``dtype`` matches the data type of the input array ``x``. If ``True``, a newly allocated array must always be returned. If ``False`` and the specified ``dtype`` matches the data type of the input array, the input array must be returned; otherwise, a newly allocated array must be returned. Default: ``True``.
     device: Optional[device]
@@ -50,7 +56,15 @@ def astype(
     Returns
     -------
     out: array
-        an array having the specified data type. The returned array must have the same shape as ``x``.
+        For ``dtype_or_kind`` a data type, an array having the specified data type.
+        For ``dtype_or_kind`` a kind of data type:
+        - If ``x.dtype`` is already of that kind, the data type is maintained.
+        - Otherwise, an attempt is made to convert to the specified kind, according to the type promotion rules (see :ref:`type-promotion`).
+            - Numeric kinds are interpreted as the lowest-precision standard data type of that kind for the purposes of type promotion.
+              For example, ``astype(x, 'complex floating')`` will return an array with the data type ``complex64`` when ``x.dtype`` is ``float32``,
+              since ``complex64`` is the result of promoting ``float32`` with the lowest-precision standard complex data type, ``complex64``.
+            - For kind ``integral``, the 'lowest-precision standard data type' is interpreted as ``int8``, not ``uint8``.
+        The returned array must have the same shape as ``x``.
 
     Notes
     -----