ENH: Add check for character limit (#57103)

Co-authored-by: Matthew Roeschke <[email protected]>

Author: luke396

doc/source/whatsnew/v3.0.0.rst

@@ -28,7 +28,7 @@ enhancement2
 
 Other enhancements
 ^^^^^^^^^^^^^^^^^^
--
+- :func:`DataFrame.to_excel` now raises an ``UserWarning`` when the character count in a cell exceeds Excel's limitation of 32767 characters (:issue:`56954`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/generic.py

@@ -2334,6 +2334,10 @@ def to_excel(
         Once a workbook has been saved it is not possible to write further
         data without rewriting the whole workbook.
 
+        pandas will check the number of rows, columns,
+        and cell character count does not exceed Excel's limitations.
+        All other limitations must be checked by the user.
+
         Examples
         --------
 

pandas/io/excel/_base.py

@@ -1326,7 +1326,15 @@ def _value_with_fmt(
             fmt = "0"
         else:
             val = str(val)
-
+            # GH#56954
+            # Excel's limitation on cell contents is 32767 characters
+            # xref https://support.microsoft.com/en-au/office/excel-specifications-and-limits-1672b34d-7043-467e-8e27-269d656771c3
+            if len(val) > 32767:
+                warnings.warn(
+                    "Cell contents too long, truncated to 32767 characters",
+                    UserWarning,
+                    stacklevel=find_stack_level(),
+                )
         return val, fmt
 
     @classmethod

pandas/tests/io/excel/test_writers.py

@@ -1385,6 +1385,18 @@ def test_to_excel_empty_frame(self, engine, ext):
             expected = DataFrame()
             tm.assert_frame_equal(result, expected)
 
+    def test_to_excel_raising_warning_when_cell_character_exceed_limit(
+        self, path, engine
+    ):
+        # GH#56954
+        df = DataFrame({"A": ["a" * 32768]})
+        msg = "Cell contents too long, truncated to 32767 characters"
+        with tm.assert_produces_warning(
+            UserWarning, match=msg, raise_on_extra_warnings=False
+        ):
+            buf = BytesIO()
+            df.to_excel(buf)
+
 
 class TestExcelWriterEngineTests:
     @pytest.mark.parametrize(