BUG: Unable to set StringArray for DataFrame with single column and dtype string

[ace-e4s]

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

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

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

Reproducible Example

import pandas as pd


df_single = pd.DataFrame(data={"a": [None, "two"]}, index=[0, 1], dtype="string")
df_multi = pd.DataFrame(data={"a": [None, "two"], "b":["1", "2"]}, index=[0, 1], dtype="string")


# ok
df_single.loc[0, ["a"]] = "one"

# ok
df_multi.loc[0, ["a"]] = "one"

# not ok
df_single.loc[0, ["a"]] = ["one"]

# ok
df_multi.loc[0, ["a"]] = ["one"]

# not ok
df_single.loc[0, ["a"]] = pd.array(["one"], dtype="string")

# ok
df_multi.loc[0, ["a"]] = pd.array(["one"], dtype="string")

Issue Description

When attempting to set a single value (string array) into an DataFrame with single column and dtype string, exception is raised: ValueError: setting an array element with a sequence.

The code example shows same operation performed different ways with a DataFrame with single and multiple columns. No issue when DataFrame has more than one column.

Expected Behavior

Set the value as in DataFrame with multiple columns.

Installed Versions

INSTALLED VERSIONS

commit : 945c9ed
python : 3.8.10.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19043
machine : AMD64
processor : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : Norwegian Bokmål_Norway.1252

pandas : 1.3.4
numpy : 1.20.3
pytz : 2021.3
dateutil : 2.8.2
pip : 21.1.3
setuptools : 58.2.0
Cython : None
pytest : 6.2.5
hypothesis : None
sphinx : 4.0.2
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.2
IPython : 7.28.0
pandas_datareader: None
bs4 : 4.9.3
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 5.0.0
pyxlsb : None
s3fs : None
scipy : 1.7.1
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : 0.54.0

[jbrockmendel]

Looks like this is driven by ExtensionBlock's special-casing of setitem, which ignores indexer[1].

Goes on the list of "will be fixed by 2D EAs"

[ace-e4s]

Is there a PR or issue I can track? Encountered this in production setting, and will be applying a hotfix while waiting on fix.

[jbrockmendel]

Is there a PR or issue I can track?

This issue right here!

Encountered this in production setting, and will be applying a hotfix while waiting on fix.

Best guess (emphasis on guess) is that a kludge could be patched into ExtensionBlock.setitem inside the if isinstance(indexer, tuple) path to unpack exactly this case.