Fix DataFrame.agg produces different types if the DataFrame is empty

[weikhor]

• [ ✔] closes BUG: DataFrame.agg produces different types if the DataFrame is empty #41672
• [ ❌] tests added / passed
• [ ❌] Ensure all linting tests pass, see here for how to run them

[pep8speaks]

Hello @weikhor! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-05-30 01:26:58 UTC

[jreback]

these go in pandas/tests/apply/test_frame_apply.py

there are already extensive tests, so find a good place to exercise the empty case

we have these, so model after

(pandas) jreback@lhs2:~/pandas-dev$ grep -r empty pandas/tests/apply/|grep def
pandas/tests/apply/test_frame_transform.py:def test_transform_empty_listlike(float_frame, ops, frame_or_series):
pandas/tests/apply/test_frame_transform.py:def test_transform_empty_dictlike(float_frame, ops, frame_or_series):
pandas/tests/apply/test_frame_transform.py:def test_transform_empty_dataframe():
pandas/tests/apply/test_series_apply.py:def test_apply_empty_integer_series_with_datetime_index():
pandas/tests/apply/test_series_apply.py:def test_map_empty(index):
pandas/tests/apply/test_frame_apply.py:def test_apply_empty(float_frame):
pandas/tests/apply/test_frame_apply.py:def test_apply_with_reduce_empty():
pandas/tests/apply/test_frame_apply.py:def test_apply_funcs_over_empty(func):
pandas/tests/apply/test_frame_apply.py:def test_nunique_empty():
pandas/tests/apply/test_frame_apply.py:def test_apply_empty_infer_type():
pandas/tests/apply/test_frame_apply.py:def test_apply_empty_list_reduce():

[github-actions]

This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this.

[simonjayhawkins]

Thanks @weikhor for the PR. can you add a release note to doc/source/whatsnew/v1.3.1.rst, Bug fixes section and address #41722 (comment)

[mroeschke]

Thanks for the PR, but it appears this has gone stale and needs addressing of the reviews. Closing, but happy to reopen if still interested.