BUG: fftn axis bug (numpy#27466)

* rfftn axis bug

* added test on shapes and fixed the linter issue

* linter length

Author: Ishankoradia

numpy/fft/_pocketfft.py

@@ -1379,7 +1379,7 @@ def rfftn(a, s=None, axes=None, norm=None, out=None):
     a = asarray(a)
     s, axes = _cook_nd_args(a, s, axes)
     a = rfft(a, s[-1], axes[-1], norm, out=out)
-    for ii in range(len(axes)-1):
+    for ii in range(len(axes)-2, -1, -1):
         a = fft(a, s[ii], axes[ii], norm, out=out)
     return a
 

numpy/fft/tests/test_pocketfft.py

@@ -307,6 +307,14 @@ def test_rfftn(self):
                         np.fft.rfftn(x, norm="ortho"), atol=1e-6)
         assert_allclose(np.fft.rfftn(x) / (30. * 20. * 10.),
                         np.fft.rfftn(x, norm="forward"), atol=1e-6)
+        # Regression test for gh-27159
+        x = np.ones((2, 3))
+        result = np.fft.rfftn(x, axes=(0, 0, 1), s=(10, 20, 40))
+        assert result.shape == (10, 21)
+        expected = np.fft.fft(np.fft.fft(np.fft.rfft(x, axis=1, n=40),
+                            axis=0, n=20), axis=0, n=10)
+        assert expected.shape == (10, 21)
+        assert_allclose(result, expected, atol=1e-6)
 
     def test_irfftn(self):
         x = random((30, 20, 10))