DOC: Exception raised if columns do not match data GH27003

[srf94]

• closes AssertionError with DataFrame.from_records() when it should create column of nulls #27003
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @srf94! 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 2019-06-26 19:58:19 UTC

[codecov]

Codecov Report

Merging #27067 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #27067 +/- ## ========================================== - Coverage 92.04% 92.03% -0.01%  ========================================== Files 180 180 Lines 50714 50714 ========================================== - Hits 46680 46676 -4  - Misses 4034 4038 +4

Flag	Coverage Δ
#multiple	90.67% <ø> (ø)	⬆️
#single	41.88% <ø> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/core/frame.py	96.89% <ø> (-0.12%)	⬇️
pandas/io/gbq.py	88.88% <0%> (-11.12%)	⬇️

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[codecov]

Codecov Report

Merging #27067 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #27067 +/- ## ========================================== - Coverage 92.04% 92.03% -0.01%  ========================================== Files 180 180 Lines 50714 50714 ========================================== - Hits 46680 46676 -4  - Misses 4034 4038 +4

Flag	Coverage Δ
#multiple	90.67% <ø> (ø)	⬆️
#single	41.88% <ø> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/core/frame.py	96.89% <ø> (-0.12%)	⬇️
pandas/io/gbq.py	88.88% <0%> (-11.12%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d94146c...d3218c7. Read the comment docs.

[WillAyd]

Can you check if we have test coverage for this scenario?

[jorisvandenbossche]

I think that we indeed first need to pin down the behaviour in code and tests. As it does not seem to always give an error message. For example:

In [24]: data = {1: ["foo"], 2: ["bar"]} In [25]: pd.DataFrame.from_records(data, columns=['a', 'b']) Out[25]: Empty DataFrame Columns: [a, b] Index: [] 

This is an example from the tests:

pandas/pandas/tests/frame/test_constructors.py

Lines 2049 to 2053 in de0867f

	data = {1: ['foo'], 2: ['bar']}
	result = DataFrame.from_records(data, columns=['a', 'b'])
	exp = DataFrame(data, columns=['a', 'b'])
	tm.assert_frame_equal(result, exp)

[jorisvandenbossche]

Also, the sentence you are changing is about the case that your data already have names. While the use case in the reported issue is when the data have no names.

[WillAyd]

This looks stale so closing for now but ping if you'd like to pick back up