BUG: `reorder_categories` with `inplace=True` is not changing the `dtype.categories`

[galipremsagar]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

In [1]: import pandas as pd

In [2]: pd.__version__
Out[2]: '1.3.2'

In [3]: sr = pd.Series(['a', 'b', 'c'], dtype='category')

In [4]: sr
Out[4]: 
0    a
1    b
2    c
dtype: category
Categories (3, object): ['a', 'b', 'c']

In [5]: new = sr.cat.reorder_categories(['c', 'b', 'a'])

In [6]: new
Out[6]: 
0    a
1    b
2    c
dtype: category
Categories (3, object): ['c', 'b', 'a']

In [7]: new.dtype.categories
Out[7]: Index(['c', 'b', 'a'], dtype='object')

In [8]: sr.cat.reorder_categories(['c', 'b', 'a'], inplace=True)
pandas/core/arrays/categorical.py:2630: FutureWarning: The `inplace` parameter in pandas.Categorical.reorder_categories is deprecated and will be removed in a future version. Removing unused categories will always return a new Categorical object.
  res = method(*args, **kwargs)

In [9]: sr
Out[9]: 
0    a
1    b
2    c
dtype: category
Categories (3, object): ['c', 'b', 'a']

In [10]: sr.dtype.categories
Out[10]: Index(['a', 'b', 'c'], dtype='object')

Problem description

When we perform reorder_categories operation as an inplace op, the categories attribute of CategoricalDtype is not updated.

Expected Output

In [7]: sr.dtype.categories
Out[7]: Index(['c', 'b', 'a'], dtype='object')

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 5f648bf
python : 3.8.10.final.0
python-bits : 64
OS : Linux
OS-release : 5.11.0-27-generic
Version : #29~20.04.1-Ubuntu SMP Wed Aug 11 15:58:17 UTC 2021
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.3.2
numpy : 1.21.2
pytz : 2021.1
dateutil : 2.8.2
pip : 21.2.4
setuptools : 57.4.0
Cython : 0.29.24
pytest : 6.2.4
hypothesis : 6.15.0
sphinx : 4.1.2
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.1
IPython : 7.26.0
pandas_datareader: None
bs4 : 4.9.3
bottleneck : None
fsspec : 2021.07.0
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 5.0.0
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : 0.53.1

[galipremsagar]

This seems to be the same for inplace operation with remove_categories as well:

In [1]: import pandas as pd

In [2]: sr = pd.Series(['a', 'b', 'c'], dtype='category')

In [3]: 

In [3]: sr
Out[3]: 
0    a
1    b
2    c
dtype: category
Categories (3, object): ['a', 'b', 'c']

In [4]: new = sr.cat.remove_categories(['a'])

In [5]: new
Out[5]: 
0    NaN
1      b
2      c
dtype: category
Categories (2, object): ['b', 'c']

In [6]: new.dtype.categories
Out[6]: Index(['b', 'c'], dtype='object')

In [7]: sr.cat.remove_categories(['a'], inplace=True)
pandas/core/arrays/categorical.py:2630: FutureWarning: The `inplace` parameter in pandas.Categorical.remove_categories is deprecated and will be removed in a future version. Removing unused categories will always return a new Categorical object.
  res = method(*args, **kwargs)

In [8]: sr
Out[8]: 
0    NaN
1      b
2      c
dtype: category
Categories (2, object): ['b', 'c']

In [9]: sr.dtype.categories
Out[9]: Index(['a', 'b', 'c'], dtype='object')

[PurnashisHazra]

take

[PurnashisHazra]

seems to be working as expected in pandas v'1.2.4'

`

  import pandas as pd
  sr = pd.Series(['a', 'b', 'c'], dtype='category')
  sr
  sr
  0    a
  1    b
  2    c
  dtype: category
  Categories (3, object): ['a', 'b', 'c']
  new = sr.cat.reorder_categories(['c', 'b', 'a'])
  new
  new
  0    a
  1    b
  2    c
  dtype: category
  Categories (3, object): ['c', 'b', 'a']
  new.dtype.categories
  Index(['c', 'b', 'a'], dtype='object')
  sr.cat.reorder_categories(['c', 'b', 'a'], inplace=True)
  sr
  sr
  0    a
  1    b
  2    c
  dtype: category
  Categories (3, object): ['c', 'b', 'a']
  sr.dtype.categories
  Index(['c', 'b', 'a'], dtype='object')
  pd.__version__
  ​
  '1.2.4'

`

[simonjayhawkins]

on 1.2.5 and earlier, sr.dtype is CategoricalDtype(categories=['c', 'b', 'a'], ordered=False)

maybe it should return an ordered categorical dtype. as CategoricalDtype(categories=['c', 'b', 'a'], ordered=False) compares equal to CategoricalDtype(categories=['a', 'b', 'c'], ordered=False)

will bisect shortly for further context.

[simonjayhawkins]

will bisect shortly for further context.

first bad commit: [c68b605] PERF: cache_readonly for Block properties (#40620)

cc @jbrockmendel

[PurnashisHazra]

will bisect shortly for further context.

first bad commit: [c68b605] PERF: cache_readonly for Block properties (#40620)

cc @jbrockmendel

could you lead me to the files that I need to look at? blocks.py? any others?

[PurnashisHazra]

changing and reverting blocks.py still gives the same error