ENH: Improve loadtxt error with dtype and non-matchinig column number (#22996)

The number "changed" is weird if the user fixed it, so give a different
message in that case.

2 files changed, 20 insertions, 2 deletions

diff --git a/numpy/core/src/multiarray/textreading/rows.c b/numpy/core/src/multiarray/textreading/rows.c
index 9c490b3f7..b8a6943dc 100644
--- a/numpy/core/src/multiarray/textreading/rows.c
+++ b/numpy/core/src/multiarray/textreading/rows.c
@@ -318,10 +318,19 @@ read_rows(stream *s,
         }
 
         if (!usecols && (actual_num_fields != current_num_fields)) {
-            PyErr_Format(PyExc_ValueError,
+            if (homogeneous) {
+                PyErr_Format(PyExc_ValueError,
                     "the number of columns changed from %zd to %zd at row %zd; "
                     "use `usecols` to select a subset and avoid this error",
                     actual_num_fields, current_num_fields, row_count+1);
+            }
+            else {
+                PyErr_Format(PyExc_ValueError,
+                    "the dtype passed requires %zd columns but %zd were found "
+                    "at row %zd; "
+                    "use `usecols` to select a subset and avoid this error",
+                    actual_num_fields, current_num_fields, row_count+1);
+            }
             goto error;
         }
 
diff --git a/numpy/lib/tests/test_loadtxt.py b/numpy/lib/tests/test_loadtxt.py
index 8a5b044b8..2d805e434 100644
--- a/numpy/lib/tests/test_loadtxt.py
+++ b/numpy/lib/tests/test_loadtxt.py
@@ -244,6 +244,14 @@ def test_converters_negative_indices_with_usecols():
                      usecols=[0, -1], converters={-1: (lambda x: -1)})
     assert_array_equal(res, [[0, -1], [0, -1]])
 
+
+def test_ragged_error():
+    rows = ["1,2,3", "1,2,3", "4,3,2,1"]
+    with pytest.raises(ValueError,
+            match="the number of columns changed from 3 to 4 at row 3"):
+        np.loadtxt(rows, delimiter=",")
+
+
 def test_ragged_usecols():
     # usecols, and negative ones, work even with varying number of columns.
     txt = StringIO("0,0,XXX\n0,XXX,0,XXX\n0,XXX,XXX,0,XXX\n")
@@ -553,7 +561,8 @@ def test_quote_support_default():
     txt = StringIO('"lat,long", 45, 30\n')
     dtype = np.dtype([('f0', 'U24'), ('f1', np.float64), ('f2', np.float64)])
 
-    with pytest.raises(ValueError, match="the number of columns changed"):
+    with pytest.raises(ValueError,
+            match="the dtype passed requires 3 columns but 4 were"):
         np.loadtxt(txt, dtype=dtype, delimiter=",")
 
     # Enable quoting support with non-None value for quotechar param