Repository: pandas-dev/pandas

Issue:
ENH: show warning when exporting dataframe and hitting excel truncating limit
### Feature Type

- [X] Adding new functionality to pandas

- [ ] Changing existing functionality in pandas

- [ ] Removing existing functionality in pandas


### Problem Description

I wish I can see a warning when I save a pandas dataframe to excel, and one of the columns contain super long strings, and these strings are truncated, possibly due to a limit on the Excel side. 

### Feature Description

detect string length and Show a warning when the some contents of a cell exceed Excel length/size limit, to let the users know that the data won't be saved fully if choose to export to excel. (csv is probably better)

### Alternative Solutions

show warning all the time or add this to pd.to_excel documentation

### Additional Context

_No response_


Discussion:
Hi @SophieGarden , could you let us know the length of strings you're dealing with? I've personally not yet encountered an issue of strings being erased in a .xlsx file (_not_ to say it can't happen), and I've dealt with exporting some pretty long strings in converting to .xlsx. Is it possible that you're running into the [visibility limit ](https://excel.tips.net/T003163_Character_Limits_for_Cells.html) for Excel, rather than the character limit?

This might be helpful to add into the pd.to_excel() documentation if this is what you're encountering. I expect others will run into the same, and if we can divert the questions to the Excel side this would probably be the best approach.
Seems like it is 32767 characters. See https://support.microsoft.com/en-au/office/excel-specifications-and-limits-1672b34d-7043-467e-8e27-269d656771c3. 

Reproducer:

```py
import pandas as pd
from io import BytesIO
df = pd.DataFrame({'col1': ['a'*35000, 'b']})
fobj = BytesIO()
df.to_excel(fobj, index=False)
fobj.seek(0)
df2 = pd.read_excel(fobj, engine='openpyxl')
assert len(df2['col1'].iloc[0]) == 35000 # raises (returns 32767 instead)
```
I think it makes sense to check the number of rows and columns (if we don't do so already). I would be opposed to checking every limitation @asishm linked to, and it does seem to me that the number of characters in a string is too detailed. That said, it appears there is a decent place for such a check in `ExcelWriter._value_with_fmt`, so I'd be okay with adding one there.

I think it would be good to add something like

> 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.

to the documentation
take

Patch:
diff --git a/doc/source/whatsnew/v3.0.0.rst b/doc/source/whatsnew/v3.0.0.rst
index 6d41c29a6c614..ec6172b4a7ba6 100644
--- a/doc/source/whatsnew/v3.0.0.rst
+++ b/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`)
 -
 
 .. ---------------------------------------------------------------------------
diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index 87a394761ee7c..9a65e53722d57 100644
--- a/pandas/core/generic.py
+++ b/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
         --------
 
diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index c7642236d4b2a..4109b6d0965bb 100644
--- a/pandas/io/excel/_base.py
+++ b/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


Test patch:
diff --git a/pandas/tests/io/excel/test_writers.py b/pandas/tests/io/excel/test_writers.py
index 6aea7464ee8dc..6ea48cd759fbc 100644
--- a/pandas/tests/io/excel/test_writers.py
+++ b/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(