Any categorical dtype object's .unique() changes categories · Issue #18291 · pandas-dev/pandas (original) (raw)

Code Sample, a copy-pastable example if possible

In [2]: pd.Categorical([1,3,2,4], categories=[4,2,3,1,5]).unique() Out[2]: [1, 3, 2, 4] Categories (4, int64): [1, 3, 2, 4]

same happens with CategoricalIndex and Series of dtype=category (as long as ordered=False).

Problem description

The general pandas approach to categoricals seems to change the .categories field as seldom as possible. So I see no reason why unique(), which by definition does not affect the set of distinct values, should change the categories (reordering them based on appearance, and dropping unused ones).

Expected Output

In [2]: pd.Categorical([1,3,2,4], categories=[4,2,3,1,5]).unique() Out[2]: [1, 3, 2, 4] Categories (4, int64): [4, 2, 3, 1, 5]

Output of pd.show_versions()

In [3]: pd.show_versions()

INSTALLED VERSIONS

commit: 63e8527
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-3-amd64
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.22.0.dev0+131.g63e8527d3
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.7.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.6
lxml: None
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1