Empty data frames not round-trippable to JSON

[ludaavics]

Code Sample

import pandas as pd
df = pd.DataFrame([], columns=['a', 'b', 'c'])
df.to_json('tmp.json', orient='table')
pd.read_json('tmp.json', orient='table')

>> KeyError: "['index' 'a' 'b' 'c'] not in index"

Problem description

Empty data frames saved as JSON fail to load back to data frames.
Quick fix: replace this line with

df = DataFrame(table['data'], columns=col_order)[col_order]

Expected Output

print(df)
Empty DataFrame
Columns: [a, b, c]
Index: []

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.0
pytest: 3.2.3
pip: 10.0.1
setuptools: 36.6.0
Cython: 0.27.2
numpy: 1.13.3
scipy: 1.0.0
pyarrow: 0.7.1
xarray: 0.9.6
IPython: 6.2.1
sphinx: 1.6.5
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: 1.5.1
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: 0.4.0
matplotlib: 2.1.0
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.0.2
lxml: None
bs4: 4.6.0
html5lib: 1.0b10
sqlalchemy: 1.2.3
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: 0.1.3
pandas_gbq: None
pandas_datareader: None

INSTALLED VERSIONS

commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.0
pytest: None
pip: 9.0.3
setuptools: 39.1.0
Cython: None
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: None
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.4
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.0.4
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[WillAyd]

Thanks for the report and investigation - care to make a PR?

[ludaavics]

Happy to.

…

On Fri, Jun 1, 2018 at 3:04 PM William Ayd ***@***.***> wrote: Thanks for the report and investigation - care to make a PR? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#21287 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABzdrLAH5ySqQHG4OiMsoRLRsNxxl5XJks5t4ZA8gaJpZM4UXHVI> .

[pyryjook]

Hi!

I actually ended up diving into this with my PR #21318. I did the changes the way @ludaavics initially suggested (BTW, thanks for heads-up with the example!). After writing a unit test for it I realised that now that the actual error is fixed the DF read back from the JSON gets a different index type than it originally had:

By using: tm.assert_frame_equal(expected, result) this is the result:

E       AssertionError: DataFrame.index are different
E
E       DataFrame.index classes are not equivalent
E       [left]:  Index([], dtype='object')
E       [right]: Float64Index([], dtype='float64')

Need to dig this a bit deeper now. Any initial thoughts on why this might happen or am I missing something?

(This is my first contribution for this project, so might be something obvious that I have not (yet) noticed)

[WillAyd]

@pyryjook for questions specific to your commits it is easier to help if you push the commit to the PR and ask the question there

[pyryjook]

Yeah, I'll do it and let's then continue there.