merge vs. join - unify parameter names

[stefansimik]

Problem description

pandas.DataFrame.merge function is conceptually simillar like pandas.DataFrame.join function.
(first one one merges on specified columns, second merges on index).

These 2 functions use various parameters to do the same thing:

• join function has 2 params: lsuffix + rsuffix
• merge function has only 1 param: suffixes

We should unify names and usage of these parameter(s).
Let's use first way or second way, and deprecate the other way.

(Personally, I prefer to have only 1 param suffixes as it is shorter and there is 1 param for 1 purpose).

Expected Output

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.4.final.0
python-bits : 64
OS : Windows
OS-release : 7
machine : AMD64
processor : Intel64 Family 6 Model 79 Stepping 1, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.None

pandas : 0.25.2
numpy : 1.16.5
pytz : 2019.3
dateutil : 2.8.0
pip : 19.2.3
setuptools : 41.4.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.4.1
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.10.3
IPython : 7.8.0
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : 4.4.1
matplotlib : 3.1.1
numexpr : None
odfpy : None
openpyxl : 3.0.0
pandas_gbq : None
pyarrow : 0.14.1
pytables : None
s3fs : None
scipy : 1.3.1
sqlalchemy : None
tables : None
xarray : None
xlrd : 1.2.0
xlwt : None
xlsxwriter : None

[Batalex]

In addition, is there a rationale for the parameters order being different?

DataFrame.merge(self, right, how='inner', on=None, ...)
DataFrame.join(self, other, on=None, how='left',...)

We should change join's signature to match merge. I would glady submit a PR.

[stefansimik]

I think consistent order of params would be more clear for everybody.
Inconsistent order just adds confusion and complexity.