BUG: read_excel() converts the string "None" in an Excel file to "NaN"

[jmcnamara]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas. (I am working on compiling and testing this.)

Reproducible Example

import pandas as pd

excel_file = "string_list.xlsx"

df_openpyxl = pd.read_excel(excel_file, engine="openpyxl")
df_calamine = pd.read_excel(excel_file, engine="calamine")

print("openpyxl engine")
print("===============")
print(df_openpyxl)

print("calamine engine")
print("===============")
print(df_calamine)

Issue Description

The attached excel file string_list.xlsx contains the following data:

  Header
0  Alone
1   Bone
2   None
3   Cone
4   Done

It looks like this:

When read with read_excel() using either the openpyxl or calamine engine it converts the string cell "None" to a NaN. The output from the above program is:

openpyxl engine
===============
  Header
0  Alone
1   Bone
2    NaN
3   Cone
4   Done
calamine engine
===============
  Header
0  Alone
1   Bone
2    NaN
3   Cone
4   Done

Note that "None" has changed to NaN.

Sample file:

string_list.xlsx

I checked openpyxl, calamine and python-calamine outside of Pandas and they each print the expected string "None".

Expected Behavior

The string "None" from an Excel file shouldn't be interpreted as Python None and/or converted to NaN.

Installed Versions

INSTALLED VERSIONS

commit : 2cc3762
python : 3.11.1
python-bits : 64
OS : Darwin
OS-release : 24.5.0
Version : Darwin Kernel Version 24.5.0: Tue Apr 22 19:53:26 PDT 2025; root:xnu-11417.121.6~2/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.3.0
numpy : 2.1.3
pytz : 2024.2
dateutil : 2.9.0.post0
pip : 25.1.1
Cython : None
sphinx : None
IPython : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : 3.1.4
lxml.etree : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : 3.1.5
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : 20.0.0
pyreadstat : None
pytest : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : 3.2.5
zstandard : None
tzdata : 2024.2
qtpy : None
pyqt5 : None

[lsgordon]

I can look into this

[asishm]

Haven't investigated, but it's likely due to the na_values / keep_default_na parameters of read_excel. I'd try with setting keep_default_na=False

[lsgordon]

Looks like that is the issue:

keep_default_na : bool, default True
    Whether or not to include the default NaN values when parsing the data.
    Depending on whether ``na_values`` is passed in, the behavior is as follows:

    * If ``keep_default_na`` is True, and ``na_values`` are specified,
      ``na_values`` is appended to the default NaN values used for parsing.
    * If ``keep_default_na`` is True, and ``na_values`` are not specified, only
      the default NaN values are used for parsing.
    * If ``keep_default_na`` is False, and ``na_values`` are specified, only
      the NaN values specified ``na_values`` are used for parsing.
    * If ``keep_default_na`` is False, and ``na_values`` are not specified, no
      strings will be parsed as NaN.

Yup, None is in there

Turning that to False should resolve the issue.

[jmcnamara]

Thanks, I can confirm that add keep_default_na=False make it work as expected.

df_openpyxl = pd.read_excel(excel_file, engine="openpyxl", keep_default_na=False)
df_calamine = pd.read_excel(excel_file, engine="calamine", keep_default_na=False)

openpyxl engine
===============
  Header
0  Alone
1   Bone
2   None
3   Cone
4   Done
calamine engine
===============
  Header
0  Alone
1   Bone
2   None
3   Cone
4   Done

So, to be clear, are you saying that this is expected behaviour and not a bug?

[asishm]

Yes, this is expected behavior.

[jmcnamara]

Yes, this is expected behavior.

Thanks. Then I will close.