BUG: series.truncate doesn't return the correct value in pandas 1.1.0

[gabicca]

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• [Y] I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Code Sample, a copy-pastable example

def test_truncate():
    series = pd.Series([0.1], index=pd.DatetimeIndex(['2020-08-04']))
    before = pd.Timestamp('2020-08-02')
    after = pd.Timestamp('2020-08-04')

    assert series.truncate(before=before, after=after).equals(series)

Problem description

The above test passes in pandas 1.0.5 while fails in pandas 1.1.0.

After diving into the code I found that truncate in pandas/core/generic.py changed as follows:

if ax.is_monotonic_decreasing:
     before, after = after, before

the above check was added to the method. ax.is_monotonic_decreasing returns true for 1-element series, (ax.is_monotonic_increasing also returns true). When before, and after are given in the correct, increasing order (like in the test), the following check reverses them and the result of truncate will be an empty series, instead of returning the input series which is the expected behaviour.

[TomAugspurger]

Thanks for the report and clear diagnosis.

Are you interested in submitting a pull request with a fix? I think a check that len(ax) > 1 could be added before flipping the before and after.

[gabicca]

Thanks for the report and clear diagnosis.

Are you interested in submitting a pull request with a fix? I think a check that len(ax) > 1 could be added before flipping the before and after.

Thanks! Sure, I'll take a closer look and submit a PR with the fix.