dtype changed when DataFrame add DateOffset

[holymonson]

Code Sample, a copy-pastable example if possible

pd.DataFrame([pd.to_datetime('2018')]).dtypes
# 0    datetime64[ns]
# dtype: object
(pd.DataFrame([pd.to_datetime('2018')]) + pd.DateOffset(years=1)).dtypes
# 0    object
# dtype: object

Problem description

Timestamp + DateOffset = Timestamp, but DataFrame(datetime64[ns]) + DateOffset = DataFrame(object).

Expected Output

DataFrame(datetime64[ns]) + DateOffset = DataFrame(datetime64[ns]). Don't change dtype.

Output of pd.show_versions()

[paste the output of pd.show_versions() here below this line]

INSTALLED VERSIONS

commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Darwin
OS-release: 17.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: zh_CN.UTF-8
LOCALE: zh_CN.UTF-8

pandas: 0.23.0
pytest: None
pip: 10.0.1
setuptools: 39.2.0
Cython: None
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: None
sphinx: 1.6.2
patsy: None
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.8.0
bs4: 4.5.3
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[chris-b1]

Yeah, I guess the dispatch on this only works for Series, this should also work.

In [19]: df[0] = df[0] + pd.DateOffset(years=1)

In [20]: df
Out[20]:
           0
0 2019-01-01

In [21]: df.dtypes
Out[21]:
0    datetime64[ns]
dtype: object

cc @jbrockmendel (anything open that already covers this?)

[jbrockmendel]

anything open that already covers this?

I don't think so. Just added a note to #18824.

[holymonson]

pandas/pandas/core/internals.py

Lines 1359 to 1369 in 322dbf4

	# coerce/transpose the args if needed
	try:
	values, values_mask, other, other_mask = self._try_coerce_args(
	transf(values), other)
	except TypeError:
	block = self.coerce_to_target_dtype(orig_other)
	return block.eval(func, orig_other,
	errors=errors,
	try_cast=try_cast, mgr=mgr)

DateOffset is dtype object, a DatetimeBlock operated with object will cast to ObjectBlock.

Besides, pd.DataFrame([pd.Timestamp('2018')]) + pd.Timedelta('1d') fails, but it seems easier to fix.