Requiring default index for to_feather() is unintuitive

[drkarthi]

Code Sample

import pandas as pd
df = pd.DataFrame({'a': [1,2,3,4], 'b': [5,6,7,8]} )
df = df.drop(1)
df.to_feather("test.feather")

Problem description

I saw documentation in the code requiring default index at:

pandas/pandas/io/feather_format.py

Lines 46 to 52 in 794be8c

	if not df.index.equals(RangeIndex.from_range(range(len(df)))):
	raise ValueError(
	"feather does not support serializing a "
	"non-default index for the index; you "
	"can .reset_index() to make the index "
	"into column(s)"
	)

However, I think it is unintuitive to require the user to have a default index before writing to feather (for example this is not a requirement to writing to csv). Why is this a requirement? What are your thoughts about reindexing being the default?

Expected Output

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.6.9.final.0
python-bits : 64
OS : Darwin
OS-release : 17.7.0
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 0.25.0
numpy : 1.17.0
pytz : 2019.2
dateutil : 2.8.0
pip : 19.2.2
setuptools : 41.0.1
Cython : None
pytest : 5.1.0
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 3.8.0
html5lib : 1.0.1
pymysql : None
psycopg2 : 2.8.3 (dt dec pq3 ext lo64)
jinja2 : 2.8.1
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : 3.8.0
matplotlib : 3.1.1
numexpr : 2.7.0
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 0.14.1
pytables : None
s3fs : None
scipy : 1.3.1
sqlalchemy : 1.3.7
tables : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None

[jreback]

feather (which is just pyarrow under the hood) does not save the index, so round-tripping fails with a non-default index. this is a design decision of the format to make it simpler. parquet does allow saving of the index.

[drkarthi]

Got it, thanks for the info

[jmakov]

@jreback would be great to have that in the docs.

[jreback]

would take a PR to update the docs

[jmakov]

Would take a sponsorship from NumFOCUS to send a PR to update the docs.

[jreback]

@jmakov not for a simple PR

[JohannesWiesner]

Would be nice though, if this was possible. It would make the alignment of several data frames more fail-safe.

[mikecoder5]

@jreback does this reason still apply? Running the sample code block from above using pyarrow.feather.write_feather() seems to now work as expected. Can we delete the default index check from Pandas feather_format.py?

[jreback]

@mikecoder5 you are welcome to submit a PR that tries that and will see what the CI says

[mikecoder5]

Quick update, pyarrow Feather doesn't seem to work for indexes that aren't a RangeIndex, filed an issue on the Apache Arrow project. If the team decides to address that request, then we can move forward with the suggestion to remove the index checks from Pandas.