DEP: deprecate scalar conversions for arrays with ndim > 0 (#10615)

This PR reflects some of the progress achieved in issue #10404 and is used to asses the impact of the changes. 

With the changes in this PR, `float(numpy.array([1.0])` now gives a warning; likewise some other things:
```python
import numpy

a = numpy.random.rand(10, 1)
a[0] = numpy.array([1.0])       # okay
a[0] = numpy.array(1.0)         # okay
a[0] = 1.0                      # okay

b = numpy.random.rand(10)
b[0] = numpy.array([1.0])       # ValueError: setting an array element with a sequence.
b[0, ...] = numpy.array([1.0])  # okay
b[0] = numpy.array(1.0)         # okay
b[0] = 1.0                      # okay
```
This aligns the behavior of numpy arrays with that of lists:
```python
float([3.14]) 
```
```
TypeError: float() argument must be a string or a number, not 'list'
```
```python
import numpy as np

a = np.random.rand(5)
a[0] = [3.14]
```
```
ValueError: setting an array element with a sequence.
```

Fixes #10404.

4 files changed, 39 insertions, 7 deletions

diff --git a/numpy/core/src/multiarray/common.c b/numpy/core/src/multiarray/common.c
index 001d299c7..573d0d606 100644
--- a/numpy/core/src/multiarray/common.c
+++ b/numpy/core/src/multiarray/common.c
@@ -441,3 +441,32 @@ new_array_for_sum(PyArrayObject *ap1, PyArrayObject *ap2, PyArrayObject* out,
     }
 }
 
+NPY_NO_EXPORT int
+check_is_convertible_to_scalar(PyArrayObject *v)
+{
+    if (PyArray_NDIM(v) == 0) {
+        return 0;
+    }
+
+    /* Remove this if-else block when the deprecation expires */
+    if (PyArray_SIZE(v) == 1) {
+        /* Numpy 1.25.0, 2023-01-02 */
+        if (DEPRECATE(
+                "Conversion of an array with ndim > 0 to a scalar "
+                "is deprecated, and will error in future. "
+                "Ensure you extract a single element from your array "
+                "before performing this operation. "
+                "(Deprecated NumPy 1.25.)") < 0) {
+            return -1;
+        }
+        return 0;
+    } else {
+        PyErr_SetString(PyExc_TypeError,
+            "only length-1 arrays can be converted to Python scalars");
+        return -1;
+    }
+
+    PyErr_SetString(PyExc_TypeError,
+            "only 0-dimensional arrays can be converted to Python scalars");
+    return -1;
+}
diff --git a/numpy/core/src/multiarray/common.h b/numpy/core/src/multiarray/common.h
index 127c6250d..cb9fadc4e 100644
--- a/numpy/core/src/multiarray/common.h
+++ b/numpy/core/src/multiarray/common.h
@@ -332,6 +332,13 @@ PyArray_TupleFromItems(int n, PyObject *const *items, int make_null_none)
     return tuple;
 }
 
+/*
+ * Returns 0 if the array has rank 0, -1 otherwise. Prints a deprecation
+ * warning for arrays of _size_ 1.
+ */
+NPY_NO_EXPORT int
+check_is_convertible_to_scalar(PyArrayObject *v);
+
 
 #include "ucsnarrow.h"
 
diff --git a/numpy/core/src/multiarray/methods.c b/numpy/core/src/multiarray/methods.c
index 93b290020..3b2e72820 100644
--- a/numpy/core/src/multiarray/methods.c
+++ b/numpy/core/src/multiarray/methods.c
@@ -2804,9 +2804,7 @@ array_complex(PyArrayObject *self, PyObject *NPY_UNUSED(args))
     PyArray_Descr *dtype;
     PyObject *c;
 
-    if (PyArray_SIZE(self) != 1) {
-        PyErr_SetString(PyExc_TypeError,
-                "only length-1 arrays can be converted to Python scalars");
+    if (check_is_convertible_to_scalar(self) < 0) {
         return NULL;
     }
 
diff --git a/numpy/core/src/multiarray/number.c b/numpy/core/src/multiarray/number.c
index c208fb203..35e4af79c 100644
--- a/numpy/core/src/multiarray/number.c
+++ b/numpy/core/src/multiarray/number.c
@@ -869,13 +869,11 @@ array_scalar_forward(PyArrayObject *v,
                      PyObject *(*builtin_func)(PyObject *),
                      const char *where)
 {
-    PyObject *scalar;
-    if (PyArray_SIZE(v) != 1) {
-        PyErr_SetString(PyExc_TypeError, "only size-1 arrays can be"\
-                        " converted to Python scalars");
+    if (check_is_convertible_to_scalar(v) < 0) {
         return NULL;
     }
 
+    PyObject *scalar;
     scalar = PyArray_GETITEM(v, PyArray_DATA(v));
     if (scalar == NULL) {
         return NULL;