BUG: shuffling empty array with axis=1 was broken

This would trigger:
```
NotImplementedError: Axis argument is only supported on ndarray objects
```
because empty arrays and array scalars would take the "untyped" path.

The bug exists only for Generator, not in RandomState (it doesn't have
axis keyword for `shuffle`), but update both because it keeps
implementations in sync and the change results in more understandable
code also for RandomState.

4 files changed, 39 insertions, 8 deletions

diff --git a/numpy/random/_generator.pyx b/numpy/random/_generator.pyx
index 5a7d4a21a..ba1713dfa 100644
--- a/numpy/random/_generator.pyx
+++ b/numpy/random/_generator.pyx
@@ -4415,7 +4415,15 @@ cdef class Generator:
                 with self.lock, nogil:
                     _shuffle_raw_wrap(&self._bitgen, n, 1, itemsize, stride,
                                       x_ptr, buf_ptr)
-        elif isinstance(x, np.ndarray) and x.ndim and x.size:
+        elif isinstance(x, np.ndarray):
+            if axis >= x.ndim:
+                raise np.AxisError(f"Cannot shuffle along axis {axis} for "
+                                   f"array of dimension {x.ndim}")
+
+            if x.size == 0:
+                # shuffling is a no-op
+                return
+
             x = np.swapaxes(x, 0, axis)
             buf = np.empty_like(x[0, ...])
             with self.lock:
@@ -4429,12 +4437,13 @@ cdef class Generator:
                     x[i] = buf
         else:
             # Untyped path.
-            if not isinstance(x, (np.ndarray, MutableSequence)):
+            if not isinstance(x, MutableSequence):
                 # See gh-18206. We may decide to deprecate here in the future.
                 warnings.warn(
                     "`x` isn't a recognized object; `shuffle` is not guaranteed "
                     "to behave correctly. E.g., non-numpy array/tensor objects "
-                    "with view semantics may contain duplicates after shuffling."
+                    "with view semantics may contain duplicates after shuffling.",
+                    UserWarning, stacklevel=2
                 )
 
             if axis != 0:
diff --git a/numpy/random/mtrand.pyx b/numpy/random/mtrand.pyx
index 9ee3d9ff3..814630c03 100644
--- a/numpy/random/mtrand.pyx
+++ b/numpy/random/mtrand.pyx
@@ -4457,7 +4457,11 @@ cdef class RandomState:
                     self._shuffle_raw(n, sizeof(np.npy_intp), stride, x_ptr, buf_ptr)
                 else:
                     self._shuffle_raw(n, itemsize, stride, x_ptr, buf_ptr)
-        elif isinstance(x, np.ndarray) and x.ndim and x.size:
+        elif isinstance(x, np.ndarray):
+            if x.size == 0:
+                # shuffling is a no-op
+                return
+
             buf = np.empty_like(x[0, ...])
             with self.lock:
                 for i in reversed(range(1, n)):
@@ -4469,12 +4473,13 @@ cdef class RandomState:
                     x[i] = buf
         else:
             # Untyped path.
-            if not isinstance(x, (np.ndarray, MutableSequence)):
+            if not isinstance(x, MutableSequence):
                 # See gh-18206. We may decide to deprecate here in the future.
                 warnings.warn(
                     "`x` isn't a recognized object; `shuffle` is not guaranteed "
                     "to behave correctly. E.g., non-numpy array/tensor objects "
-                    "with view semantics may contain duplicates after shuffling."
+                    "with view semantics may contain duplicates after shuffling.",
+                    UserWarning, stacklevel=2
                 )
 
             with self.lock:
diff --git a/numpy/random/tests/test_generator_mt19937.py b/numpy/random/tests/test_generator_mt19937.py
index c4fb5883c..47c81584c 100644
--- a/numpy/random/tests/test_generator_mt19937.py
+++ b/numpy/random/tests/test_generator_mt19937.py
@@ -960,6 +960,14 @@ class TestRandomDist:
         random.shuffle(actual, axis=-1)
         assert_array_equal(actual, desired)
 
+    def test_shuffle_custom_axis_empty(self):
+        random = Generator(MT19937(self.seed))
+        desired = np.array([]).reshape((0, 6))
+        for axis in (0, 1):
+            actual = np.array([]).reshape((0, 6))
+            random.shuffle(actual, axis=axis)
+            assert_array_equal(actual, desired)
+
     def test_shuffle_axis_nonsquare(self):
         y1 = np.arange(20).reshape(2, 10)
         y2 = y1.copy()
@@ -993,6 +1001,11 @@ class TestRandomDist:
         arr = [[1, 2, 3], [4, 5, 6]]
         assert_raises(NotImplementedError, random.shuffle, arr, 1)
 
+        arr = np.array(3)
+        assert_raises(TypeError, random.shuffle, arr)
+        arr = np.ones((3, 2))
+        assert_raises(np.AxisError, random.shuffle, arr, 2)
+
     def test_permutation(self):
         random = Generator(MT19937(self.seed))
         alist = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
@@ -1004,7 +1017,7 @@ class TestRandomDist:
         arr_2d = np.atleast_2d([1, 2, 3, 4, 5, 6, 7, 8, 9, 0]).T
         actual = random.permutation(arr_2d)
         assert_array_equal(actual, np.atleast_2d(desired).T)
-        
+
         bad_x_str = "abcd"
         assert_raises(np.AxisError, random.permutation, bad_x_str)
 
diff --git a/numpy/random/tests/test_randomstate.py b/numpy/random/tests/test_randomstate.py
index b70a04347..7f5f08050 100644
--- a/numpy/random/tests/test_randomstate.py
+++ b/numpy/random/tests/test_randomstate.py
@@ -642,7 +642,7 @@ class TestRandomDist:
         a = np.array([42, 1, 2])
         p = [None, None, None]
         assert_raises(ValueError, random.choice, a, p=p)
-    
+
     def test_choice_p_non_contiguous(self):
         p = np.ones(10) / 5
         p[1::2] = 3.0
@@ -699,6 +699,10 @@ class TestRandomDist:
             assert_equal(
                 sorted(b.data[~b.mask]), sorted(b_orig.data[~b_orig.mask]))
 
+        def test_shuffle_invalid_objects(self):
+            x = np.array(3)
+            assert_raises(TypeError, random.shuffle, x)
+
     def test_permutation(self):
         random.seed(self.seed)
         alist = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]