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Bug fix one element series truncate #35547

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Merged
merged 5 commits into from
Aug 7, 2020
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Bug fix one element series truncate #35547

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8f91dd0
BUG: series.truncate fix for one-element series (#35544)
gabicca Aug 4, 2020
a72a62b
BUG: series.truncate fix for one-element series (#35544) - black form…
gabicca Aug 4, 2020
f667212
BUG: series.truncate fix for one-element series (#35544) - updated re…
gabicca Aug 4, 2020
2e85aef
BUG: series.truncate fix for one-element series (#35544) - PR review …
gabicca Aug 5, 2020
215e65d
Merge remote-tracking branch 'upstream/master' into bug-fix-one-eleme…
gabicca Aug 7, 2020
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2 changes: 1 addition & 1 deletion doc/source/whatsnew/v1.1.1.rst
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Expand Up @@ -48,7 +48,7 @@ Categorical

**Indexing**

-
- Bug in :meth:`Series.truncate` when trying to truncate a single-element series (:issue:`35544`)

**DataFrame**
- Bug in :class:`DataFrame` constructor failing to raise ``ValueError`` in some cases when data and index have mismatched lengths (:issue:`33437`)
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2 changes: 1 addition & 1 deletion pandas/core/generic.py
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Expand Up @@ -9405,7 +9405,7 @@ def truncate(
if before > after:
raise ValueError(f"Truncate: {after} must be after {before}")

if ax.is_monotonic_decreasing:
if len(ax) > 1 and ax.is_monotonic_decreasing:
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Is there a reason why a single element container should be monotonic_decrasing? Wondering if the real issue isn't that property

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Both .is_monotonic_decreasing and .is_monotonic_increasing will return true for a single-element series. I don't think checking whether a single element series is "increasing" or "decreasing" really makes sense, so this fix really just prevents the code from checking.

The fix was suggested by @TomAugspurger , and the test is passing after the fix.

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I don't think checking whether a single element series is "increasing" or "decreasing" really makes sense

Yea that's my point. Instead of special casing this to check the length if the property returned False you would get the same result

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OK, misunderstood your point. I'll take a look at that code.

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@TomAugspurger TomAugspurger Aug 4, 2020 •
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I think length-1 (and probably length-0) sequences are both monotonically increasing and decreasing by definition.

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I think length-1 (and probably length-0) sequences are both monotonically increasing and decreasing by definition.

After digging a bit, I think you're right, though couldn't find a really good reference... Do you maybe have one?

before, after = after, before

slicer = [slice(None, None)] * self._AXIS_LEN
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11 changes: 11 additions & 0 deletions pandas/tests/series/methods/test_truncate.py
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Expand Up @@ -141,3 +141,14 @@ def test_truncate_multiindex(self):
expected = df.col

tm.assert_series_equal(result, expected)

def test_truncate_one_element_series(self):
# GH 35544
series = pd.Series([0.1], index=pd.DatetimeIndex(["2020-08-04"]))
before = pd.Timestamp("2020-08-02")
after = pd.Timestamp("2020-08-04")

result = series.truncate(before=before, after=after)

# the input Series and the expected Series are the same
tm.assert_series_equal(result, series)