BUG: union of lists of DatetimeIndex generates ValueErrors

[btodac]

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Reproducible Example

import pandas as pd
a = pd.date_range(start=pd.Timestamp.today() - pd.Timedelta(days=30), end=pd.Timestamp.today())
b = [a[:5], a[5:10], a[5:7], a[10:]]
b[0].union(b[1:])
# and
#b[0].union(b[1:2])

Issue Description

The first example fails with ValueError: Lengths must match to compare, when performing a monotonic data check. (pandas/core/indexes/base.py", line 6672, in _cmp_method raise ValueError("Lengths must match to compare")
The second example fails with ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all(), when performing a monotonic data check (File "pandas/_libs/algos.pyx", line 754, in pandas._libs.algos.is_monotonic).

union_many() works fine but generates a soon to be deprecated warning

Expected Behavior

import pandas as pd
a = pd.date_range(start=pd.Timestamp.today() - pd.Timedelta(days=30), end=pd.Timestamp.today())
b = [a[:5], a[5:10], a[5:7], a[10:]]
b[0].union_many(b[1:])

Installed Versions

INSTALLED VERSIONS

commit : 06d2301
python : 3.10.4.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.0-122-generic
Version : #138-Ubuntu SMP Wed Jun 22 15:00:31 UTC 2022
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 1.4.1
numpy : 1.22.3
pytz : 2022.1
dateutil : 2.8.2
pip : 22.0.4
setuptools : 60.10.0
Cython : None
pytest : None
hypothesis : None
sphinx : 4.4.0
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.8.0
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.0
IPython : 7.32.0
pandas_datareader: 0.10.0
bs4 : 4.10.0
bottleneck : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.5.1
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.8.0
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
zstandard : None

[pratyushsharan]

I won't say this is a bug, as union was supposed to work with DatetimeIndex only, rather than list[DatetimeIndex]. For that purpose, union_many is supposed to be used, which will be deprecated as you observed. Maybe use functools.reduce?
reduce(lambda x,y: x.union(y), b)

[btodac]

Is the plan to remove the functionality of union_many when it is deprecated or is union going to be expanded to incorporate the functionality?

[phofl]

union_many is a wrapper around union, so there is no performance impact when removing this.

[phofl]

Closing. Please ping, if there is anything we are missing

[btodac]

I don't understand the response. Either union_many should be maintained or union should be adapted to provide the same usage as union_many. Otherwise you are reducing the usability of the package. To be clear:
If you are deprecating union_many you should not advise users to use obj.union in the deprecation warning as this does not offer the same functionality

[phofl]

You can try append.

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