Skip to content

Stable Sorting Algorithm for Fillna Indexer #21212

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 3 commits into from
May 29, 2018
Merged

Stable Sorting Algorithm for Fillna Indexer #21212

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
be005f0
Added failing test case
WillAyd May 25, 2018
5f8fdf4
Changed argort kind argument in fillna_indexer
WillAyd May 25, 2018
617ae08
LINT fixup and whatsnew
WillAyd May 25, 2018
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
1 change: 1 addition & 0 deletions doc/source/whatsnew/v0.23.1.txt
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -52,6 +52,7 @@ Groupby/Resample/Rolling
^^^^^^^^^^^^^^^^^^^^^^^^

- Bug in :func:`DataFrame.agg` where applying multiple aggregation functions to a :class:`DataFrame` with duplicated column names would cause a stack overflow (:issue:`21063`)
- Bug in :func:`pandas.core.groupby.GroupBy.ffill` and :func:`pandas.core.groupby.GroupBy.bfill` where the fill within a grouping would not always be applied as intended due to the implementations' use of a non-stable sort (:issue:`21207`)

Strings
^^^^^^^
Expand Down
3 changes: 2 additions & 1 deletion pandas/_libs/groupby.pyx
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -297,7 +297,8 @@ def group_fillna_indexer(ndarray[int64_t] out, ndarray[int64_t] labels,
# Make sure all arrays are the same size
assert N == len(labels) == len(mask)

sorted_labels = np.argsort(labels).astype(np.int64, copy=False)
sorted_labels = np.argsort(labels, kind='mergesort').astype(
np.int64, copy=False)
if direction == 'bfill':
sorted_labels = sorted_labels[::-1]

Expand Down
17 changes: 17 additions & 0 deletions pandas/tests/groupby/test_transform.py
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -721,6 +721,23 @@ def interweave(list_obj):
assert_frame_equal(result, exp)


@pytest.mark.parametrize("fill_method", ['ffill', 'bfill'])
def test_pad_stable_sorting(fill_method):
# GH 21207
x = [0] * 20
y = [np.nan] * 10 + [1] * 10

if fill_method == 'bfill':
y = y[::-1]

df = pd.DataFrame({'x': x, 'y': y})
expected = df.copy()

result = getattr(df.groupby('x'), fill_method)()

tm.assert_frame_equal(result, expected)


@pytest.mark.parametrize("test_series", [True, False])
@pytest.mark.parametrize("periods,fill_method,limit", [
(1, 'ffill', None), (1, 'ffill', 1),
Expand Down