testing.assert_frame_equal obj parameter not always respected

[ajwood]

Code Sample

import pandas as pd

df = pd.DataFrame({'a': [1,2], 'b': [2,3]})

df_coldiff = pd.DataFrame({'a': [1,2], 'ZZZ': [2,3]})
pd.testing.assert_frame_equal(df, df_coldiff, obj='myobj')
# raises: 
#  AssertionError: myobj.columns are different
#  
#  myobj.columns values are different (50.0 %)
#  [left]:  Index(['a', 'b'], dtype='object')
#  [right]: Index(['a', 'ZZZ'], dtype='object')


df_valuediff = pd.DataFrame({'a': [1,2], 'b': [2,999]})
pd.testing.assert_frame_equal(df, df_valuediff, obj='myobj')
# raises:
#  AssertionError: DataFrame.iloc[:, 1] are different
#  
#  DataFrame.iloc[:, 1] values are different (50.0 %)
#  [left]:  [2, 3]
#  [right]: [2, 999]

Problem description

In the first example where the column names differ, the 'myobj' label is used in the assertion message.

In the second example where the values differ, the 'myobj' label does not appear.

It looks to me like these strings should be formatted with obj instead of hardcoded with DataFrame?

https://github.com/pandas-dev/pandas/blob/master/pandas/util/testing.py#L1252
https://github.com/pandas-dev/pandas/blob/master/pandas/util/testing.py#L1267

Expected Output

I expected the assertion message for the second example to be:

#  AssertionError: myobj.iloc[:, 1] are different
#  
#  myobj.iloc[:, 1] values are different (50.0 %)
#  [left]:  [2, 3]
#  [right]: [2, 999]

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.7.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-52-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.24.2
pytest: 4.4.1
pip: 19.1
setuptools: 41.0.1
Cython: 0.29.7
numpy: 1.16.3
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.5.0
sphinx: 2.0.1
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: 1.2.1
tables: 3.5.1
numexpr: 2.6.9
feather: None
matplotlib: 3.0.3
openpyxl: 2.6.1
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.7
lxml.etree: 4.3.3
bs4: 4.7.1
html5lib: 1.0.1
sqlalchemy: 1.3.5
pymysql: None
psycopg2: None
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

[WillAyd]

I don't think this is really intended as part of the API but sure seems like a reasonable change. Want to make a PR?

[ajwood]

Sure, I can issue a PR. It's my first time pulling/building/running the pandas project though, so it might take me a while to figure it out.

I don't think this is really intended as part of the API

Out of curiosity, could you elaborate? Am I looking at this the wrong way, if I see this as a clear bug in the assertion message?

[WillAyd]

Out of curiosity, could you elaborate?

I'm just looking at the docstring:

Specify object name being compared, internally used to show appropriate
assertion message.

So never really intended for general purpose usage but I think simple enough to change

based off research you've already done

Sure, I can issue a PR. It's my first time pulling/building/running the pandas project though, so it might take me a while to figure it out.

Be sure to check out our contributing guide if you have not already:

https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#contributing-to-pandas

[ajwood]

Thanks, looking at it now. Do you think the commit message prefix for this change should be TST or BUG?

[WillAyd]

TST works