BUG: Empty dataframe created with Index, not RangeIndex

[connesy]

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

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

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

# Your code here
>>> import pandas as pd
>>> df = pd.DataFrame(columns=['a', 'b'])
>>> print(df.index)
Index([], dtype='object')

>>> df2 = df.append(df, ignore_index=True)
>>> print(df2.index)
RangeIndex(start=0, stop=0, step=1)

>>> pd.testing.assert_frame_equal(df, df2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/stefan/miniconda3/envs/pandas/lib/python3.9/site-packages/pandas/_testing.py", line 1657, in assert_frame_equal
    assert_index_equal(
  File "/home/stefan/miniconda3/envs/pandas/lib/python3.9/site-packages/pandas/_testing.py", line 773, in assert_index_equal
    _check_types(left, right, obj=obj)
  File "/home/stefan/miniconda3/envs/pandas/lib/python3.9/site-packages/pandas/_testing.py", line 740, in _check_types
    assert_class_equal(left, right, exact=exact, obj=obj)
  File "/home/stefan/miniconda3/envs/pandas/lib/python3.9/site-packages/pandas/_testing.py", line 868, in assert_class_equal
    raise_assert_detail(obj, msg, repr_class(left), repr_class(right))
  File "/home/stefan/miniconda3/envs/pandas/lib/python3.9/site-packages/pandas/_testing.py", line 1073, in raise_assert_detail
    raise AssertionError(msg)
AssertionError: DataFrame.index are different

DataFrame.index classes are not equivalent
[left]:  Index([], dtype='object')
[right]: RangeIndex(start=0, stop=0, step=1)

Problem description

When creating an empty dataframe it is created with Index, and not with a RangeIndex as specified in the docs. This is a problem, e.g. in the example shown above, where I would not expect the index to change when appending the empty dataframe to itself. However, pd.testing.assert_frame_equals raises AssertionError since the indeces are not equivalent.

Expected Output

>>> df = pd.DataFrame(columns=['a', 'b'])
>>> print(df.index)
RangeIndex(start=0, stop=0, step=1)

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 7d32926
python : 3.9.2.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.0-59-generic
Version : #65-Ubuntu SMP Thu Dec 10 12:01:51 UTC 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.2.2
numpy : 1.20.1
pytz : 2021.1
dateutil : 2.8.1
pip : 21.0.1
setuptools : 49.6.0.post20210108
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[alexprincel]

The following code in pandas.core.internals.construction seems to be the culprit. I do not think I have enough maturity with the Pandas API to understand the code-breaking potential of editing the following from Index to RangeIndex or to reorganize the code without the length check:

def extract_index(data) -> Index:
    """
    Try to infer an Index from the passed data, raise ValueError on failure.
    """
    index = None
    if len(data) == 0:
        index = Index([])
    elif len(data) > 0:
        # ...

    return ensure_index(index)

[mzeitlin11]

Persists on current master, contributions welcome to fix this!