BUG: Appending empty list to DataFrame #28769

[asishm]

• closes DataFrame.append with empty list raises IndexError #28769
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[simonjayhawkins]

@asishm

from the docs..

other : DataFrame or Series/dict-like object, or list of these
The data to append.

shouldn't passing an empty list raise an Exception. but perhaps with a more user friendly message.

[asishm]

@simonjayhawkins
That sounds reasonable. The reason for implementing it to return the same dataframe was based on the suggestion in #28769. I could update the docs to mention that if other is an empty list, then it would return the same dataframe. Or maybe explicitly state that passing an empty list would result in an Exception (ValueError?)

[simonjayhawkins]

I'm not sure. see what others think.

not raising on an empty list is perhaps more pythonic, but without checking, I'm not sure how we handle this scenario in other methods.

It maybe worth adding type annotations to append and see what mypy has to offer.

[jreback]

can you parameterize with empty list, tuple, np.ndarray

[asishm]

@jreback Please see the reply in the other comment (#28834 (comment))

[pep8speaks]

Hello @asishm! 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 2019-11-16 20:08:13 UTC

[jreback]

can you merge master

[asishm]

Rebased master. I'm not sure if there's an issue with the CI or if I messed something up

[asishm]

Rebased master. I'm not sure if there's an issue with the CI or if I messed something up

I see this in the Travis logs
https://travis-ci.org/pandas-dev/pandas/jobs/612167009#L2418

psql: could not connect to server: No such file or directory
	Is the server running locally and accepting
	connections on Unix domain socket "/tmp/.s.PGSQL.5432"?
The command "ci/setup_env.sh" failed and exited with 2 during .

[jreback]

thanks @asishm

[joaoe]

@asishm

shouldn't passing an empty list raise an Exception. but perhaps with a more user friendly message.

That would be quite special behavior:

123 + 0 == 123
x = []; x.extend([]); x == []
"a" + "" == "a"

empty lists, strings, etc should be neutral in operations instead of causing errors. That saves an unecessary if check for a corner case.