Merge pull request #20016 from seberg/issue-20009

BUG: Add a warning for user dtypes modifying casts after use

1 files changed, 63 insertions, 4 deletions

diff --git a/numpy/core/src/multiarray/usertypes.c b/numpy/core/src/multiarray/usertypes.c
index d0cf53576..a338d712d 100644
--- a/numpy/core/src/multiarray/usertypes.c
+++ b/numpy/core/src/multiarray/usertypes.c
@@ -268,6 +268,56 @@ PyArray_RegisterDataType(PyArray_Descr *descr)
     return typenum;
 }
 
+
+/*
+ * Checks that there is no cast already cached using the new casting-impl
+ * mechanism.
+ * In that case, we do not clear out the cache (but otherwise silently
+ * continue).  Users should not modify casts after they have been used,
+ * but this may also happen accidentally during setup (and may never have
+ * mattered).  See https://github.com/numpy/numpy/issues/20009
+ */
+static int _warn_if_cast_exists_already(
+        PyArray_Descr *descr, int totype, char *funcname)
+{
+    PyArray_DTypeMeta *to_DType = PyArray_DTypeFromTypeNum(totype);
+    if (to_DType == NULL) {
+        return -1;
+    }
+    PyObject *cast_impl = PyDict_GetItemWithError(
+            NPY_DT_SLOTS(NPY_DTYPE(descr))->castingimpls, (PyObject *)to_DType);
+    Py_DECREF(to_DType);
+    if (cast_impl == NULL) {
+        if (PyErr_Occurred()) {
+            return -1;
+        }
+    }
+    else {
+        char *extra_msg;
+        if (cast_impl == Py_None) {
+            extra_msg = "the cast will continue to be considered impossible.";
+        }
+        else {
+            extra_msg = "the previous definition will continue to be used.";
+        }
+        Py_DECREF(cast_impl);
+        PyArray_Descr *to_descr = PyArray_DescrFromType(totype);
+        int ret = PyErr_WarnFormat(PyExc_RuntimeWarning, 1,
+                "A cast from %R to %R was registered/modified using `%s` "
+                "after the cast had been used.  "
+                "This registration will have (mostly) no effect: %s\n"
+                "The most likely fix is to ensure that casts are the first "
+                "thing initialized after dtype registration.  "
+                "Please contact the NumPy developers with any questions!",
+                descr, to_descr, funcname, extra_msg);
+        Py_DECREF(to_descr);
+        if (ret < 0) {
+            return -1;
+        }
+    }
+    return 0;
+}
+
 /*NUMPY_API
   Register Casting Function
   Replaces any function currently stored.
@@ -279,14 +329,19 @@ PyArray_RegisterCastFunc(PyArray_Descr *descr, int totype,
     PyObject *cobj, *key;
     int ret;
 
-    if (totype < NPY_NTYPES_ABI_COMPATIBLE) {
-        descr->f->cast[totype] = castfunc;
-        return 0;
-    }
     if (totype >= NPY_NTYPES && !PyTypeNum_ISUSERDEF(totype)) {
         PyErr_SetString(PyExc_TypeError, "invalid type number.");
         return -1;
     }
+    if (_warn_if_cast_exists_already(
+            descr, totype, "PyArray_RegisterCastFunc") < 0) {
+        return -1;
+    }
+
+    if (totype < NPY_NTYPES_ABI_COMPATIBLE) {
+        descr->f->cast[totype] = castfunc;
+        return 0;
+    }
     if (descr->f->castdict == NULL) {
         descr->f->castdict = PyDict_New();
         if (descr->f->castdict == NULL) {
@@ -328,6 +383,10 @@ PyArray_RegisterCanCast(PyArray_Descr *descr, int totype,
                         "RegisterCanCast must be user-defined.");
         return -1;
     }
+    if (_warn_if_cast_exists_already(
+            descr, totype, "PyArray_RegisterCanCast") < 0) {
+        return -1;
+    }
 
     if (scalar == NPY_NOSCALAR) {
         /*