API: Always use BufferError when dlpack export fails

See also data-apis/array-api#498.

I think we should just change this.  It is a niche feature and just
an error type change.

Closes gh-20742

doc/release/upcoming_changes/22542.compatibility.rst

@@ -0,0 +1,7 @@
+DLPack export raises ``BufferError``
+------------------------------------
+When an array buffer cannot be exported via DLPack a
+``BufferError`` is now always raised where previously
+``TypeError`` or ``RuntimeError`` was raised.
+This allows falling back to the buffer protocol or
+``__array_interface__`` when DLPack was tried first.

numpy/core/src/multiarray/dlpack.c

@@ -133,14 +133,15 @@ array_dlpack(PyArrayObject *self,
     }
 
     if (stream != Py_None) {
-        PyErr_SetString(PyExc_RuntimeError, "NumPy only supports "
-                "stream=None.");
+        PyErr_SetString(PyExc_RuntimeError,
+                "NumPy only supports stream=None.");
         return NULL;
     }
 
     if ( !(PyArray_FLAGS(self) & NPY_ARRAY_WRITEABLE)) {
-        PyErr_SetString(PyExc_TypeError, "NumPy currently only supports "
-                "dlpack for writeable arrays");
+        PyErr_SetString(PyExc_BufferError,
+            "Cannot export readonly array since signalling readonly "
+            "is unsupported by DLPack.");
         return NULL;
     }
 
@@ -152,7 +153,7 @@ array_dlpack(PyArrayObject *self,
     if (!PyArray_IS_C_CONTIGUOUS(self) && PyArray_SIZE(self) != 1) {
         for (int i = 0; i < ndim; ++i) {
             if (shape[i] != 1 && strides[i] % itemsize != 0) {
-                PyErr_SetString(PyExc_RuntimeError,
+                PyErr_SetString(PyExc_BufferError,
                         "DLPack only supports strides which are a multiple of "
                         "itemsize.");
                 return NULL;
@@ -164,8 +165,8 @@ array_dlpack(PyArrayObject *self,
     PyArray_Descr *dtype = PyArray_DESCR(self);
 
     if (PyDataType_ISBYTESWAPPED(dtype)) {
-        PyErr_SetString(PyExc_TypeError, "DLPack only supports native "
-                    "byte swapping.");
+        PyErr_SetString(PyExc_BufferError,
+                "DLPack only supports native byte order.");
             return NULL;
     }
 
@@ -182,8 +183,9 @@ array_dlpack(PyArrayObject *self,
         // We can't be sure that the dtype is
         // IEEE or padded.
         if (itemsize > 8) {
-            PyErr_SetString(PyExc_TypeError, "DLPack only supports IEEE "
-                    "floating point types without padding.");
+            PyErr_SetString(PyExc_BufferError,
+                    "DLPack only supports IEEE floating point types "
+                    "without padding (longdouble typically is not IEEE).");
             return NULL;
         }
         managed_dtype.code = kDLFloat;
@@ -192,16 +194,17 @@ array_dlpack(PyArrayObject *self,
         // We can't be sure that the dtype is
         // IEEE or padded.
         if (itemsize > 16) {
-            PyErr_SetString(PyExc_TypeError, "DLPack only supports IEEE "
-                    "complex point types without padding.");
+            PyErr_SetString(PyExc_BufferError,
+                    "DLPack only supports IEEE floating point types "
+                    "without padding (longdouble typically is not IEEE).");
             return NULL;
         }
         managed_dtype.code = kDLComplex;
     }
     else {
-        PyErr_SetString(PyExc_TypeError,
-                        "DLPack only supports signed/unsigned integers, float "
-                        "and complex dtypes.");
+        PyErr_SetString(PyExc_BufferError,
+                "DLPack only supports signed/unsigned integers, float "
+                "and complex dtypes.");
         return NULL;
     }
 

numpy/core/tests/test_dlpack.py

@@ -26,7 +26,7 @@ def test_strides_not_multiple_of_itemsize(self):
         y = np.zeros((5,), dtype=dt)
         z = y['int']
 
-        with pytest.raises(RuntimeError):
+        with pytest.raises(BufferError):
             np.from_dlpack(z)
 
     @pytest.mark.skipif(IS_PYPY, reason="PyPy can't get refcounts.")
@@ -53,14 +53,14 @@ def test_dtype_passthrough(self, dtype):
     def test_invalid_dtype(self):
         x = np.asarray(np.datetime64('2021-05-27'))
 
-        with pytest.raises(TypeError):
+        with pytest.raises(BufferError):
             np.from_dlpack(x)
 
     def test_invalid_byte_swapping(self):
         dt = np.dtype('=i8').newbyteorder()
         x = np.arange(5, dtype=dt)
 
-        with pytest.raises(TypeError):
+        with pytest.raises(BufferError):
             np.from_dlpack(x)
 
     def test_non_contiguous(self):
@@ -100,15 +100,15 @@ def dlpack_deleter_exception(self):
         x = np.arange(5)
         _ = x.__dlpack__()
         raise RuntimeError
-    
+
     def test_dlpack_destructor_exception(self):
         with pytest.raises(RuntimeError):
             self.dlpack_deleter_exception()
 
     def test_readonly(self):
         x = np.arange(5)
         x.flags.writeable = False
-        with pytest.raises(TypeError):
+        with pytest.raises(BufferError):
             x.__dlpack__()
 
     def test_ndim0(self):