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.

7 files changed, 69 insertions, 16 deletions

diff --git a/numpy/core/function_base.py b/numpy/core/function_base.py
index c82b495b1..00e4e6b0e 100644
--- a/numpy/core/function_base.py
+++ b/numpy/core/function_base.py
@@ -168,7 +168,7 @@ def linspace(start, stop, num=50, endpoint=True, retstep=False, dtype=None,
     y += start
 
     if endpoint and num > 1:
-        y[-1] = stop
+        y[-1, ...] = stop
 
     if axis != 0:
         y = _nx.moveaxis(y, 0, axis)
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;
diff --git a/numpy/core/tests/test_deprecations.py b/numpy/core/tests/test_deprecations.py
index b92a20a12..e47a24995 100644
--- a/numpy/core/tests/test_deprecations.py
+++ b/numpy/core/tests/test_deprecations.py
@@ -822,6 +822,18 @@ class TestLoadtxtParseIntsViaFloat(_DeprecationTestCase):
                 assert isinstance(e.__cause__, DeprecationWarning)
 
 
+class TestScalarConversion(_DeprecationTestCase):
+    # 2023-01-02, 1.25.0
+    def test_float_conversion(self):
+        self.assert_deprecated(float, args=(np.array([3.14]),))
+
+    def test_behaviour(self):
+        b = np.array([[3.14]])
+        c = np.zeros(5)
+        with pytest.warns(DeprecationWarning):
+            c[0] = b
+
+
 class TestPyIntConversion(_DeprecationTestCase):
     message = r".*stop allowing conversion of out-of-bound.*"
 
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index 4a064827d..984047c87 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -3644,9 +3644,13 @@ class TestMethods:
             msg = 'dtype: {0}'.format(dt)
             ap = complex(a)
             assert_equal(ap, a, msg)
-            bp = complex(b)
+
+            with assert_warns(DeprecationWarning):
+                bp = complex(b)
             assert_equal(bp, b, msg)
-            cp = complex(c)
+
+            with assert_warns(DeprecationWarning):
+                cp = complex(c)
             assert_equal(cp, c, msg)
 
     def test__complex__should_not_work(self):
@@ -3669,7 +3673,8 @@ class TestMethods:
         assert_raises(TypeError, complex, d)
 
         e = np.array(['1+1j'], 'U')
-        assert_raises(TypeError, complex, e)
+        with assert_warns(DeprecationWarning):
+            assert_raises(TypeError, complex, e)
 
 class TestCequenceMethods:
     def test_array_contains(self):
@@ -8756,8 +8761,10 @@ class TestConversion:
         int_funcs = (int, lambda x: x.__int__())
         for int_func in int_funcs:
             assert_equal(int_func(np.array(0)), 0)
-            assert_equal(int_func(np.array([1])), 1)
-            assert_equal(int_func(np.array([[42]])), 42)
+            with assert_warns(DeprecationWarning):
+                assert_equal(int_func(np.array([1])), 1)
+            with assert_warns(DeprecationWarning):
+                assert_equal(int_func(np.array([[42]])), 42)
             assert_raises(TypeError, int_func, np.array([1, 2]))
 
             # gh-9972
@@ -8772,7 +8779,8 @@ class TestConversion:
                     def __trunc__(self):
                         return 3
                 assert_equal(3, int_func(np.array(HasTrunc())))
-                assert_equal(3, int_func(np.array([HasTrunc()])))
+                with assert_warns(DeprecationWarning):
+                    assert_equal(3, int_func(np.array([HasTrunc()])))
             else:
                 pass
 
@@ -8781,8 +8789,9 @@ class TestConversion:
                     raise NotImplementedError
             assert_raises(NotImplementedError,
                 int_func, np.array(NotConvertible()))
-            assert_raises(NotImplementedError,
-                int_func, np.array([NotConvertible()]))
+            with assert_warns(DeprecationWarning):
+                assert_raises(NotImplementedError,
+                    int_func, np.array([NotConvertible()]))
 
 
 class TestWhere: