Description
Code Sample, a copy-pastable example if possible
test = pd.DataFrame(
{
'foo' : ['small', 'large', 'large', 'large', 'medium', 'large', 'large', 'medium'],
'bar' : ['C', 'A', 'A', 'C', 'A', 'C', 'A', 'C']
})
test['foo'] = test['foo'].astype('category').cat.set_categories(['tiny','small', 'medium', 'large'], ordered=True)
test.groupby(['bar', 'foo']).size().unstack()
# output
foo medium large small
bar
A 1.0 3.0 NaN
C 1.0 2.0 1.0Problem description
I have a dataframe with an ordered category column foo. I want to group by both columns then take the size of the groups and unstack to get a summary table. If all of the values in my ordered category are in the data, then the result is as expected:
test = pd.DataFrame(
{
'foo' : ['small', 'large', 'large', 'large', 'medium', 'large', 'large', 'medium'],
'bar' : ['C', 'A', 'A', 'C', 'A', 'C', 'A', 'C']
})
test['foo'] = test['foo'].astype('category').cat.set_categories(['small', 'medium', 'large'], ordered=True)
print(test.groupby(['bar', 'foo']).size().unstack())
# output
foo small medium large
bar
A NaN 1.0 3.0
C 1.0 1.0 2.0My columns appear in the specified order. However, if for some reason I have categories that are listed but don't actually appear in the data (in this case, 'tiny') the order seems to be determined by the order that the categories appear in the series before stacking:
test = pd.DataFrame(
{
'foo' : ['small', 'large', 'large', 'large', 'medium', 'large', 'large', 'medium'],
'bar' : ['C', 'A', 'A', 'C', 'A', 'C', 'A', 'C']
})
test['foo'] = test['foo'].astype('category').cat.set_categories(['small', 'medium', 'large'], ordered=True)
print(test.groupby(['bar', 'foo']).size())
print(test.groupby(['bar', 'foo']).size().unstack())
# output
bar foo
A medium 1
large 3
C small 1
medium 1
large 2
dtype: int64
foo medium large small
bar
A 1.0 3.0 NaN
C 1.0 2.0 1.0I originally encountered this when using pd.cut to group rows into bins, but an explicitly ordered category I thought made a clearer example. It's also very easy to end up in this situation when filtering a large dataframe.
Expected Output
foo small medium large
bar
A NaN 1.0 3.0
C 1.0 1.0 2.0Output of pd.show_versions()
[paste the output of pd.show_versions() here below this line]
INSTALLED VERSIONS
commit: None
python: 3.7.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-64-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8
pandas: 0.24.2
pytest: None
pip: 19.1.1
setuptools: 41.0.1
Cython: None
numpy: 1.16.4
scipy: 1.3.0
pyarrow: None
xarray: None
IPython: 7.5.0
sphinx: None
patsy: None
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.1.0
openpyxl: None
xlrd: 1.2.0
xlwt: None
xlsxwriter: None
lxml.etree: 4.3.3
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None