Inconsistent behaviour when apply() used on categorical with NaN values

[manuhortet]

• closes Inconsistent behaviour when calling apply() on a categorical column with missing data  #20714
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[jschendel]

Can you use the result/expected variable naming convention for this test, e.g.

s = pd.Series(['1-1', '1-1', np.NaN], dtype='category')
result = s.apply(lambda x: x.split('-')[0])
expected = pd.Series(['1', '1', np.NaN], dtype='category')

Additionally, the second example in the issue is the one that needs to be tested: pd.Series(['1-1','1-2',np.NaN], dtype='category'). The example you're testing is already working, so it doesn't test if the bug was actually fixed. Perhaps you could parametrize over the values of the Series and test both since expected is the same in both cases, e.g.

@pytest.mark.parametrize('values', [
    ['1-1', '1-1', np.NaN],
    ['1-1', '1-2', np.NaN]])
def test_apply_categorical_with_nan_values(self, values):
    # GH 20714
    s = pd.Series(values, dtype='category')
    ...

[manuhortet]

Thanks for the indications @jschendel !

Anyway I found another problem, doing:

@pytest.mark.parametrize('values', [
    ['1-1', '1-1', np.NaN],
    ['1-1', '1-2', np.NaN]])
def test_apply_categorical_with_nan_values(self, values):
s = pd.Series(values, dtype='category')
result = s.apply(lambda x: x.split('-')[0])
expected = pd.Series(['1', '1', np.NaN], dtype='category')
tm.assert_series_equal(result, expected, check_dtype=False)

I get an error due to the different classes of result and expected:

AssertionError: Categorical Expected type <class 'pandas.core.arrays.categorical.Categorical'>, found <class 'numpy.ndarray'> instead

This situation was already commented on issue #20714 and remarked as out of scope for this PR.
Knowing that, this test should pass. I thought I could do .astype('category') on result, but maybe I'm just missing another assert function inside tm that will work for this. Any ideas?

[jreback]

can you merge master and update

[alimcmaster1]

@manuhortet do you still want to take this on? @jreback if we don't hear back I will take this

[jreback]

@manuhortet or @alimcmaster1 if interested pls merge master

[jreback]

merge master and update

[jreback]

closing as stale if you want to continue working, please ping.