pd.cut() with user-specified labels and bins arguments does not use user-provided labels

Code Sample

import pandas as pd

bins = pd.IntervalIndex.from_tuples([(0, 2), (2, 2.5), (2.5, 5)])
pd.cut([1, 1.5, 2, 2.5, 3, 3.5], bins=bins, labels=['a', 'b', 'c']).tolist()

# output of pd.cut(...) is:
# [Interval(0.0, 2.0, closed='right'), Interval(0.0, 2.0, closed='right'), Interval(0.0, 2.0, closed='right'), Interval(2.0, 2.5, closed='right'), Interval(2.5, 5.0, closed='right'), Interval(2.5, 5.0, closed='right')]

Problem description

When labels are provided as an argument to pd.cut() with user-specified bins, then the output does not use the labels argument.

Expected Output

['a', 'a', 'b', 'b', 'c', 'c']

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None python: 2.7.13.final.0 python-bits: 64 OS: Darwin OS-release: 17.5.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: None.None

pandas: 0.23.0 pytest: 3.0.5 pip: 10.0.1 setuptools: 39.0.1 Cython: 0.25.2 numpy: 1.14.2 scipy: 1.0.1 pyarrow: None xarray: None IPython: 5.3.0 sphinx: None patsy: 0.5.0 dateutil: 2.7.2 pytz: 2018.3 blosc: None bottleneck: None tables: None numexpr: None feather: None matplotlib: 2.2.2 openpyxl: None xlrd: None xlwt: None xlsxwriter: None lxml: None bs4: 4.6.0 html5lib: 0.9999999 sqlalchemy: None pymysql: 0.7.11.None psycopg2: None jinja2: 2.9.5 s3fs: None fastparquet: None pandas_gbq: None pandas_datareader: None

[Ankan1991]

Just in case someone is looking for a workaround , you can create a helper dictionary and get the values: For the mentioned list:

d = dict(zip(bins,['a','b','c']))
[d.get(i) 
   for i in pd.cut([1, 1.5, 2, 2.5, 3, 3.5], bins=bins, labels=['a', 'b', 'c']).tolist()]

For a pandas series , you can use series.map:

d = dict(zip(bins,['a','b','c']))
pd.cut(series,bins).map(d)

[TomAugspurger]

Did you see the docstring for pd.cut?

labels : array or bool, optional
    Specifies the labels for the returned bins. Must be the same length as
    the resulting bins. If False, returns only integer indicators of the
    bins. This affects the type of the output container (see below).
    This argument is ignored when `bins` is an IntervalIndex.

I’m not sure exactly why, but labels is ignored for II bins.