Resampler.nunique counting data more than once

[jcrist]

xref addtl example in #13795

Pandas Resampler.nunique appears to be putting the same data in multiple bins:

import pandas as pd

# Create a series with a datetime index
index = pd.date_range('1-1-2000', '2-15-2000', freq='h')
index2 = pd.date_range('4-15-2000', '5-15-2000', freq='h')
index3 = index.append(index2)
s = pd.Series(range(len(index3)), index=index3)

# Since all elements are unique, `count` and `nunique` should give the same result
count = s.resample('M').count()
nunique = s.resample('M').nunique()

In pandas 0.18.1 and 0.18.0 these don't give the same results, when they should

In [3]: count
Out[3]:
2000-01-31    744
2000-02-29    337
2000-03-31      0
2000-04-30    384
2000-05-31    337
Freq: M, dtype: int64

In [4]: nunique
Out[4]:
2000-01-31    337
2000-02-29    744
2000-03-31      0
2000-04-30    744
2000-05-31    337
Freq: M, dtype: int64

In pandas 0.17.0 and 0.17.1 (adjusting to old style resample syntax), the nunique one fails due to a "ValueError: Wrong number of items passed 4, placement implies 5" somewhere in the depths of internals.py. If I go back to 0.16.2, I do get the same result for each.

I'm not sure what's going on here. Since the nunique results sum to larger than the length, it appears data is being counted more than once.

---

In [19]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 23.0.0
Cython: None
numpy: 1.10.4
scipy: None
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None

[jcrist]

May be related to #10914.

[jcrist]

Interestingly everything seems to work fine if agg is used with pd.Series.nunique instead:

In [11]: r = s.resample('M')

In [12]: r.agg(pd.Series.nunique)
Out[12]:
2000-01-31    744
2000-02-29    337
2000-03-31      0
2000-04-30    384
2000-05-31    337
Freq: M, dtype: int64

In [13]: r.nunique()    # same result as r.agg('nunique')
Out[13]:
2000-01-31    337
2000-02-29    744
2000-03-31      0
2000-04-30    744
2000-05-31    337
Freq: M, dtype: int64

[sinhrks]

CC: @behzadnouri

[mgalbright]

I think the root cause of the problem is in groupby.nunique(), which I believe is eventually is called by resample.nunique(). Note that groupby.nunique() has the same bug:

import pandas as pd
from pandas import Timestamp

data = ['1', '2', '3']
time = time = [Timestamp('2016-06-28 09:35:35'), Timestamp('2016-06-28 16:09:30'), Timestamp('2016-06-28 16:46:28')]
test = pd.DataFrame({'time':time, 'data':data})

#wrong counts
print test.set_index('time').groupby(pd.TimeGrouper(freq='h'))['data'].nunique(), "\n"
#correct counts
print test.set_index('time').groupby(pd.TimeGrouper(freq='h'))['data'].apply(pd.Series.nunique)

This gives

time  
2016-06-28 09:00:00    1  
2016-06-28 10:00:00    0  
2016-06-28 11:00:00    0  
2016-06-28 12:00:00    0  
2016-06-28 13:00:00    0  
2016-06-28 14:00:00    0  
2016-06-28 15:00:00    0  
2016-06-28 16:00:00    1  
Freq: H, Name: data, dtype: int64   

time  
2016-06-28 09:00:00    1  
2016-06-28 10:00:00    0  
2016-06-28 11:00:00    0  
2016-06-28 12:00:00    0  
2016-06-28 13:00:00    0  
2016-06-28 14:00:00    0  
2016-06-28 15:00:00    0  
2016-06-28 16:00:00    2  
Freq: H, Name: data, dtype: int64  

I believe the problem is in the second to last line of groupby.nunique(),
i.e. line 2955 in groupby.py

res[ids] = out

I suspect ids should not be used for the indexing--it has different dimensions than out.

pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.19.0
nose: 1.3.7
pip: 8.1.2
setuptools: 27.2.0
Cython: 0.24.1
numpy: 1.11.2
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.4.6
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: 1.1.0
tables: 3.2.3.1
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.42.0
pandas_datareader: 0.2.1

[jreback]

@mgalbright why don't you submit a pull-request with your test examples (and those from the issue), and the proposed fix. See if that breaks anything else. Would be greatly appreciated!

[aiguofer]

Hey, is there any advancement on this? I just realized that a report that I've been building is giving the wrong results and I believe it's due to this. I can't share all the code but here's a comparison of groupby.unique and groupby.nunique:

In [216]: ents.groupby(pd.Grouper(freq='1W-SAT', key='startdate'))['ent_id'].unique().tail(1)
Out[216]: 

startdate
2016-11-12    [550A00000033DHUIA2]
Freq: W-SAT, Name: ent_id, dtype: object

In [217]: ents.groupby(pd.Grouper(freq='1W-SAT', key='startdate'))['ent_id'].nunique().tail(1)
Out[217]: 

startdate
2016-11-12    7
Freq: W-SAT, Name: ent_id, dtype: int64

In [218]: ents.groupby(pd.Grouper(freq='1W-SAT', key='startdate'))['ent_id'].count().tail(1)
Out[221]: 

startdate
2016-11-12    1
Freq: W-SAT, Name: ent_id, dtype: int64

[jreback]

@aiguofer pull-requests are welcome to fix.

[hantusk]

Not really adding anything, but I just ran into this issue for a work report as well (pandas version 0.19.2). Passing to .agg(pd.Series.nunique) works great - thanks for the tip

[aiguofer]

Took a look at @mgalbright coment and suggestion and if I'm understanding the code correctly, the above PR should fix it. I ran nosetests pandas/tests/groupby and only had one unrelated test fail (test_series_groupby_value_counts() takes exactly 2 arguments (0 given)).