ERR non-ISO formats don't show position of error

doc/source/whatsnew/v2.0.0.rst

@@ -98,6 +98,7 @@ Other enhancements
 - Added :meth:`Index.infer_objects` analogous to :meth:`Series.infer_objects` (:issue:`50034`)
 - Added ``copy`` parameter to :meth:`Series.infer_objects` and :meth:`DataFrame.infer_objects`, passing ``False`` will avoid making copies for series or columns that are already non-object or where no better dtype can be inferred (:issue:`50096`)
 - :meth:`DataFrame.plot.hist` now recognizes ``xlabel`` and ``ylabel`` arguments (:issue:`49793`)
+- Improved error message in :func:`to_datetime` for non-ISO8601 formats, informing users about the position of the first error (:issue:`50361`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/_libs/tslibs/strptime.pyx

@@ -396,7 +396,8 @@ def array_strptime(
 
             result_timezone[i] = tz
 
-        except (ValueError, OutOfBoundsDatetime):
+        except (ValueError, OutOfBoundsDatetime) as ex:
+            ex.args = (str(ex) + f" at position {i}", )
             if is_coerce:
                 iresult[i] = NPY_NAT
                 continue

pandas/tests/io/parser/test_parse_dates.py

@@ -1717,7 +1717,10 @@ def test_parse_multiple_delimited_dates_with_swap_warnings():
     # GH46210
     with pytest.raises(
         ValueError,
-        match=r"^time data '31/05/2000' does not match format '%m/%d/%Y' \(match\)$",
+        match=(
+            r"^time data '31/05/2000' does not match format '%m/%d/%Y' \(match\) "
+            "at position 1$"
+        ),
     ):
         pd.to_datetime(["01/01/2000", "31/05/2000", "31/05/2001", "01/02/2000"])
 

pandas/tests/tools/test_to_datetime.py

@@ -1093,7 +1093,10 @@ def test_datetime_bool_arrays_mixed(self, cache):
             to_datetime([False, datetime.today()], cache=cache)
         with pytest.raises(
             ValueError,
-            match=r"^time data 'True' does not match format '%Y%m%d' \(match\)$",
+            match=(
+                r"^time data 'True' does not match format '%Y%m%d' "
+                r"\(match\) at position 1$"
+            ),
         ):
             to_datetime(["20130101", True], cache=cache)
         tm.assert_index_equal(
@@ -2072,7 +2075,9 @@ def test_to_datetime_on_datetime64_series(self, cache):
     def test_to_datetime_with_space_in_series(self, cache):
         # GH 6428
         ser = Series(["10/18/2006", "10/18/2008", " "])
-        msg = r"^time data ' ' does not match format '%m/%d/%Y' \(match\)$"
+        msg = (
+            r"^time data ' ' does not match format '%m/%d/%Y' \(match\) at position 2$"
+        )
         with pytest.raises(ValueError, match=msg):
             to_datetime(ser, errors="raise", cache=cache)
         result_coerce = to_datetime(ser, errors="coerce", cache=cache)
@@ -2342,7 +2347,10 @@ def test_dayfirst_warnings_invalid_input(self):
 
         with pytest.raises(
             ValueError,
-            match=r"time data '03/30/2011' does not match format '%d/%m/%Y' \(match\)$",
+            match=(
+                r"time data '03/30/2011' does not match format '%d/%m/%Y' "
+                r"\(match\) at position 1$"
+            ),
         ):
             to_datetime(arr, dayfirst=True)
 
@@ -2923,17 +2931,22 @@ def test_incorrect_value_exception(self):
                 to_datetime(["today", "yesterday"])
 
     @pytest.mark.parametrize(
-        "format, warning", [(None, UserWarning), ("%Y-%m-%d %H:%M:%S", None)]
+        "format, warning",
+        [
+            (None, UserWarning),
+            ("%Y-%m-%d %H:%M:%S", None),
+            ("%Y-%d-%m %H:%M:%S", None),
+        ],
     )
     def test_to_datetime_out_of_bounds_with_format_arg(self, format, warning):
         # see gh-23830
         msg = (
-            "Out of bounds nanosecond timestamp: 2417-10-27 00:00:00 "
-            "present at position 0"
+            r"Out of bounds nanosecond timestamp: 2417-10-10 00:00:00"
+            r".* at position 0"
         )
         with pytest.raises(OutOfBoundsDatetime, match=msg):
             with tm.assert_produces_warning(warning, match="Could not infer format"):
-                to_datetime("2417-10-27 00:00:00", format=format)
+                to_datetime("2417-10-10 00:00:00", format=format)
 
     @pytest.mark.parametrize(
         "arg, origin, expected_str",