Add check for character limmit

Author: luke396

pandas/core/generic.py

@@ -2363,6 +2363,10 @@ 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

@@ -1326,6 +1326,12 @@ def _value_with_fmt(
             fmt = "0"
         else:
             val = str(val)
+            # GH#56954
+            if len(val) > 32767:
+                warnings.warn(
+                    "String value too long, truncated to 32767 characters",
+                    UserWarning,
+                    stacklevel=find_stack_level())
 
         return val, fmt
 

pandas/tests/io/excel/test_writers.py

@@ -1505,3 +1505,12 @@ def test_subclass_attr(klass):
     attrs_base = {name for name in dir(ExcelWriter) if not name.startswith("_")}
     attrs_klass = {name for name in dir(klass) if not name.startswith("_")}
     assert not attrs_base.symmetric_difference(attrs_klass)
+
+
+def test_to_excel_raising_warning_when_cell_character_exceed_limit():
+    # GH#56954
+    df = DataFrame({"A": ["a" * 32768]})
+    msg = "String value too long, truncated to 32767 characters"
+    with tm.assert_produces_warning(UserWarning, match=msg):
+        buf = BytesIO()
+        df.to_excel(buf)