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Bug in DataFrame.drop_duplicates for empty DataFrame throws error #22394

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Merged
merged 14 commits into from
Aug 23, 2018
Merged

Bug in DataFrame.drop_duplicates for empty DataFrame throws error #22394

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03af0c1
Bug in :meth:`DataFrame.drop_duplicates`for empty DataFrame throws er…
HyunTruth Aug 17, 2018
79ee155
fixed what's new to render well
HyunTruth Aug 17, 2018
31f6099
Applied changes according to reviews by @jreback
HyunTruth Aug 18, 2018
6eb53f6
removed an additional line
HyunTruth Aug 18, 2018
de745bb
changed test to accomodate the column behavior in selection
HyunTruth Aug 19, 2018
3a5d97d
Parameterized the tests
Aug 22, 2018
1f58a85
Parameterized the tests
Aug 22, 2018
4f299c5
Merge branch 'dropdup' of https://github.com/HyunTruth/pandas into dr…
Aug 22, 2018
cab0958
Adhere to flake8
Aug 22, 2018
68d69db
switched df
Aug 22, 2018
fb8845d
Try catching for empty dataframes and return self
HyunTruth Aug 22, 2018
1f12545
change requested applied
HyunTruth Aug 22, 2018
20c03ef
added inplace=True tests
HyunTruth Aug 22, 2018
fc61899
rectified inplace test to reflect actual usage
HyunTruth Aug 22, 2018
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2 changes: 1 addition & 1 deletion doc/source/whatsnew/v0.24.0.txt
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Expand Up @@ -711,7 +711,7 @@ Reshaping
- Bug in :func:`get_dummies` with Unicode attributes in Python 2 (:issue:`22084`)
- Bug in :meth:`DataFrame.replace` raises ``RecursionError`` when replacing empty lists (:issue:`22083`)
- Bug in :meth:`Series.replace` and meth:`DataFrame.replace` when dict is used as the `to_replace` value and one key in the dict is is another key's value, the results were inconsistent between using integer key and using string key (:issue:`20656`)
-
- Bug in :meth:`DataFrame.drop_duplicates` for empty ``DataFrame`` which incorrectly raises error (:issue:`20516`)
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raises an error


Build Changes
^^^^^^^^^^^^^
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3 changes: 3 additions & 0 deletions pandas/core/frame.py
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Expand Up @@ -4369,6 +4369,9 @@ def duplicated(self, subset=None, keep='first'):
from pandas.core.sorting import get_group_index
from pandas._libs.hashtable import duplicated_int64, _SIZE_HINT_LIMIT

if self.empty:
return Series()

def f(vals):
labels, shape = algorithms.factorize(
vals, size_hint=min(len(self), _SIZE_HINT_LIMIT))
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15 changes: 15 additions & 0 deletions pandas/tests/frame/test_duplicates.py
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Expand Up @@ -263,6 +263,21 @@ def test_drop_duplicates_tuple():
tm.assert_frame_equal(result, expected)


@pytest.mark.parametrize('df', [
DataFrame(),
DataFrame(columns=[]),
DataFrame(columns=['A', 'B', 'C']),
DataFrame(index=[]),
DataFrame(index=['A', 'B', 'C'])
])
def test_drop_duplicates_empty(df):
# GH 20516
result = df.drop_duplicates()
if df.columns.empty is False:
df = DataFrame(columns=[])
tm.assert_frame_equal(result, df)
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This tests feels a bit tricky to me. So, when there are columns, we set them to an empty list. So, that would be the same as simply using as expected value DataFrame(columns=[]), or am I missing something?

But not sure if I'm missing something, but I'd expect DataFrame(columns=['A', 'B', 'C']).drop_duplicates() to keep the columns. Any reason to drop them?

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@HyunTruth HyunTruth Aug 22, 2018 •
edited
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Please check the closed issue of #22409. DataFrame(columns=['A', 'B', 'C']).drop_duplicates() passes through empty selection, and whilst index remains, column infos do not as the selection itself is done on the column basis if I understood the comment correctly.

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I don't think the problem may be your approach implementing this story. I agree on what @WillAyd said in #22409, but not in the side effect in this PR.

I guess df[pandas.Series()] is the same as df[[]], so you're telling the DataFrame to select an empty list of columns, so none of them. So, if we have a DataFrame with 4 columns and 10 rows, I expect this to return the 10 rows, but 0 columns.

But the case here with drop_duplicates, if I understand correctly, is that you have a DataFrame with 4 columns and 0 rows, you remove the duplicates, that there are none, as the DataFrame has 0 rows, so I should still have 4 columns and 0 rows.

And in your test you're asserting that the return will be 0 columns and 0 rows.

Am I right? Does it makes sense?

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@HyunTruth HyunTruth Aug 22, 2018 •
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Yeah. However, if we want to leave the column when there exists no value, then we'd need to change the DataFrame.drop_duplicates itself to not use the logic of self[~duplicated], as it currently does, as this is what is causing this problem at least at this moment with the empty return of DataFrame.duplicated... I thought that DataFrame.duplicated returns a Series, and then if DataFrame has no value, returning an empty Series is logical. However, since DataFrame.drop_duplicates use the logic of self[~duplicated] (which works for every other case except on empty DataFrame with only columns) returning empty Series leads to df[[]], resulting in a DataFrame with no value without the columns (as it is used in selection which has no value in it). Maybe returning an empty wasn't a good idea. Completely stuck then.

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Oh, I understand the problem now. There is some magic on df[something] in pandas. In some cases something would be understood as the list of columns, like in df[[]] or df[['A', 'B', 'C']], but if something is a list of boolean values, then it filters on rows instead of columns, like in df[df['A'].isnull()] (df['A'].isnull() returns a list (Series) of booleans).

That's why your approach is not working as expected. You may try to check with df.iloc[something], which should always filter on rows.

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can you add a test case with inplace=True please?

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Sure.



def test_drop_duplicates_NA():
# none
df = DataFrame({'A': [None, None, 'foo', 'bar',
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