BUG: groupby.rank results do not scale to 100% for dense

[peterpanmj]

Code Sample

In [61]: df_test = pd.DataFrame({"A":[1,1,2,2],"B":[1,1,1,1]})

In [62]: df_test.groupby("B").rank(method="dense", ascending=True, pct=False, na_option='top')
Out[62]:
     A
0  1.0
1  1.0
2  2.0
3  2.0

In [63]: df_test.groupby("B").rank(method="dense", ascending=True, pct=True, na_option='top')
Out[63]:
      A
0  0.25
1  0.25
2  0.50
3  0.50

Problem description

pd.groupby.rank result does not scale to 100% when method is "dense".

Expected Output

In [65]: df_test['A'].rank(method="dense", ascending=True, pct=True, na_option='top')
Out[65]:
0    0.5
1    0.5
2    1.0
3    1.0
Name: A, dtype: float64

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: 448124c138dc39001638aacd68f253b1034d7f04 python: 3.6.4.final.0 python-bits: 64 OS: Windows OS-release: 7 machine: AMD64 processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel byteorder: little LC_ALL: None LANG: zh_CN.UTF-8 LOCALE: None.None

pandas: 0.23.0.dev0+743.g448124c13
pytest: 3.3.2
pip: 9.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.7
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.4.10
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.1
pymysql: 0.7.11.None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[peterpanmj]

cc @rouzazari

[jreback]

xref #15639

[peterpanmj]

I am working on a fix. There are some existing tests do not expect the percentage ranks to scale to 1. We need to agree on what is expected results first.

pandas/pandas/tests/groupby/test_rank.py

Lines 200 to 204 in 791de95

	('dense', True, 'no_na', True,
	[0.125, 0.125, 0.5, 0.375, 0.125, 0.25, 0.5, 0.5]),
	('dense', False, 'no_na', False, [3., 3., 4., 1., 3., 2., 4., 4.]),
	('dense', False, 'no_na', True,
	[0.375, 0.375, 0.5, 0.125, 0.375, 0.25, 0.5, 0.5])

#20781 #19481
@jreback @WillAyd

[WillAyd]

Hmm didn't realize that's how things worked outside of GroupBy. If I'm not mistaken all of the values here should just be doubled