/pandas/tests/groupby/test_nth.py

import numpy as np
import pandas as pd
from pandas import DataFrame, MultiIndex, Index, Series, isna
from pandas.compat import lrange
from pandas.util.testing import (
    assert_frame_equal,
    assert_produces_warning,
    assert_series_equal)

from .common import MixIn


class TestNth(MixIn):

    def test_first_last_nth(self):
        # tests for first / last / nth
        grouped = self.df.groupby('A')
        first = grouped.first()
        expected = self.df.loc[[1, 0], ['B', 'C', 'D']]
        expected.index = Index(['bar', 'foo'], name='A')
        expected = expected.sort_index()
        assert_frame_equal(first, expected)

        nth = grouped.nth(0)
        assert_frame_equal(nth, expected)

        last = grouped.last()
        expected = self.df.loc[[5, 7], ['B', 'C', 'D']]
        expected.index = Index(['bar', 'foo'], name='A')
        assert_frame_equal(last, expected)

        nth = grouped.nth(-1)
        assert_frame_equal(nth, expected)

        nth = grouped.nth(1)
        expected = self.df.loc[[2, 3], ['B', 'C', 'D']].copy()
        expected.index = Index(['foo', 'bar'], name='A')
        expected = expected.sort_index()
        assert_frame_equal(nth, expected)

        # it works!
        grouped['B'].first()
        grouped['B'].last()
        grouped['B'].nth(0)

        self.df.loc[self.df['A'] == 'foo', 'B'] = np.nan
        assert isna(grouped['B'].first()['foo'])
        assert isna(grouped['B'].last()['foo'])
        assert isna(grouped['B'].nth(0)['foo'])

        # v0.14.0 whatsnew
        df = DataFrame([[1, np.nan], [1, 4], [5, 6]], columns=['A', 'B'])
        g = df.groupby('A')
        result = g.first()
        expected = df.iloc[[1, 2]].set_index('A')
        assert_frame_equal(result, expected)

        expected = df.iloc[[1, 2]].set_index('A')
        result = g.nth(0, dropna='any')
        assert_frame_equal(result, expected)

    def test_first_last_nth_dtypes(self):

        df = self.df_mixed_floats.copy()
        df['E'] = True
        df['F'] = 1

        # tests for first / last / nth
        grouped = df.groupby('A')
        first = grouped.first()
        expected = df.loc[[1, 0], ['B', 'C', 'D', 'E', 'F']]
        expected.index = Index(['bar', 'foo'], name='A')
        expected = expected.sort_index()
        assert_frame_equal(first, expected)

        last = grouped.last()
        expected = df.loc[[5, 7], ['B', 'C', 'D', 'E', 'F']]
        expected.index = Index(['bar', 'foo'], name='A')
        expected = expected.sort_index()
        assert_frame_equal(last, expected)

        nth = grouped.nth(1)
        expected = df.loc[[3, 2], ['B', 'C', 'D', 'E', 'F']]
        expected.index = Index(['bar', 'foo'], name='A')
        expected = expected.sort_index()
        assert_frame_equal(nth, expected)

        # GH 2763, first/last shifting dtypes
        idx = lrange(10)
        idx.append(9)
        s = Series(data=lrange(11), index=idx, name='IntCol')
        assert s.dtype == 'int64'
        f = s.groupby(level=0).first()
        assert f.dtype == 'int64'

    def test_nth(self):
        df = DataFrame([[1, np.nan], [1, 4], [5, 6]], columns=['A', 'B'])
        g = df.groupby('A')

        assert_frame_equal(g.nth(0), df.iloc[[0, 2]].set_index('A'))
        assert_frame_equal(g.nth(1), df.iloc[[1]].set_index('A'))
        assert_frame_equal(g.nth(2), df.loc[[]].set_index('A'))
        assert_frame_equal(g.nth(-1), df.iloc[[1, 2]].set_index('A'))
        assert_frame_equal(g.nth(-2), df.iloc[[0]].set_index('A'))
        assert_frame_equal(g.nth(-3), df.loc[[]].set_index('A'))
        assert_series_equal(g.B.nth(0), df.set_index('A').B.iloc[[0, 2]])
        assert_series_equal(g.B.nth(1), df.set_index('A').B.iloc[[1]])
        assert_frame_equal(g[['B']].nth(0),
                           df.loc[[0, 2], ['A', 'B']].set_index('A'))

        exp = df.set_index('A')
        assert_frame_equal(g.nth(0, dropna='any'), exp.iloc[[1, 2]])
        assert_frame_equal(g.nth(-1, dropna='any'), exp.iloc[[1, 2]])

        exp['B'] = np.nan
        assert_frame_equal(g.nth(7, dropna='any'), exp.iloc[[1, 2]])
        assert_frame_equal(g.nth(2, dropna='any'), exp.iloc[[1, 2]])

        # out of bounds, regression from 0.13.1
        # GH 6621
        df = DataFrame({'color': {0: 'green',
                                  1: 'green',
                                  2: 'red',
                                  3: 'red',
                                  4: 'red'},
                        'food': {0: 'ham',
                                 1: 'eggs',
                                 2: 'eggs',
                                 3: 'ham',
                                 4: 'pork'},
                        'two': {0: 1.5456590000000001,
                                1: -0.070345000000000005,
                                2: -2.4004539999999999,
                                3: 0.46206000000000003,
                                4: 0.52350799999999997},
                        'one': {0: 0.56573799999999996,
                                1: -0.9742360000000001,
                                2: 1.033801,
                                3: -0.78543499999999999,
                                4: 0.70422799999999997}}).set_index(['color',
                                                                     'food'])

        result = df.groupby(level=0, as_index=False).nth(2)
        expected = df.iloc[[-1]]
        assert_frame_equal(result, expected)

        result = df.groupby(level=0, as_index=False).nth(3)
        expected = df.loc[[]]
        assert_frame_equal(result, expected)

        # GH 7559
        # from the vbench
        df = DataFrame(np.random.randint(1, 10, (100, 2)), dtype='int64')
        s = df[1]
        g = df[0]
        expected = s.groupby(g).first()
        expected2 = s.groupby(g).apply(lambda x: x.iloc[0])
        assert_series_equal(expected2, expected, check_names=False)
        assert expected.name == 1
        assert expected2.name == 1

        # validate first
        v = s[g == 1].iloc[0]
        assert expected.iloc[0] == v
        assert expected2.iloc[0] == v

        # this is NOT the same as .first (as sorted is default!)
        # as it keeps the order in the series (and not the group order)
        # related GH 7287
        expected = s.groupby(g, sort=False).first()
        result = s.groupby(g, sort=False).nth(0, dropna='all')
        assert_series_equal(result, expected)

        # doc example
        df = DataFrame([[1, np.nan], [1, 4], [5, 6]], columns=['A', 'B'])
        g = df.groupby('A')
        # PR 17493, related to issue 11038
        # test Series.nth with True for dropna produces DeprecationWarning
        with assert_produces_warning(FutureWarning):
            result = g.B.nth(0, dropna=True)
        expected = g.B.first()
        assert_series_equal(result, expected)

        # test multiple nth values
        df = DataFrame([[1, np.nan], [1, 3], [1, 4], [5, 6], [5, 7]],
                       columns=['A', 'B'])
        g = df.groupby('A')

        assert_frame_equal(g.nth(0), df.iloc[[0, 3]].set_index('A'))
        assert_frame_equal(g.nth([0]), df.iloc[[0, 3]].set_index('A'))
        assert_frame_equal(g.nth([0, 1]), df.iloc[[0, 1, 3, 4]].set_index('A'))
        assert_frame_equal(
            g.nth([0, -1]), df.iloc[[0, 2, 3, 4]].set_index('A'))
        assert_frame_equal(
            g.nth([0, 1, 2]), df.iloc[[0, 1, 2, 3, 4]].set_index('A'))
        assert_frame_equal(
            g.nth([0, 1, -1]), df.iloc[[0, 1, 2, 3, 4]].set_index('A'))
        assert_frame_equal(g.nth([2]), df.iloc[[2]].set_index('A'))
        assert_frame_equal(g.nth([3, 4]), df.loc[[]].set_index('A'))

        business_dates = pd.date_range(start='4/1/2014', end='6/30/2014',
                                       freq='B')
        df = DataFrame(1, index=business_dates, columns=['a', 'b'])
        # get the first, fourth and last two business days for each month
        key = (df.index.year, df.index.month)
        result = df.groupby(key, as_index=False).nth([0, 3, -2, -1])
        expected_dates = pd.to_datetime(
            ['2014/4/1', '2014/4/4', '2014/4/29', '2014/4/30', '2014/5/1',
             '2014/5/6', '2014/5/29', '2014/5/30', '2014/6/2', '2014/6/5',
             '2014/6/27', '2014/6/30'])
        expected = DataFrame(1, columns=['a', 'b'], index=expected_dates)
        assert_frame_equal(result, expected)

    def test_nth_multi_index(self):
        # PR 9090, related to issue 8979
        # test nth on MultiIndex, should match .first()
        grouped = self.three_group.groupby(['A', 'B'])
        result = grouped.nth(0)
        expected = grouped.first()
        assert_frame_equal(result, expected)

    def test_nth_multi_index_as_expected(self):
        # PR 9090, related to issue 8979
        # test nth on MultiIndex
        three_group = DataFrame(
            {'A': ['foo', 'foo', 'foo', 'foo', 'bar', 'bar', 'bar', 'bar',
                   'foo', 'foo', 'foo'],
             'B': ['one', 'one', 'one', 'two', 'one', 'one', 'one', 'two',
                   'two', 'two', 'one'],
             'C': ['dull', 'dull', 'shiny', 'dull', 'dull', 'shiny', 'shiny',
                   'dull', 'shiny', 'shiny', 'shiny']})
        grouped = three_group.groupby(['A', 'B'])
        result = grouped.nth(0)
        expected = DataFrame(
            {'C': ['dull', 'dull', 'dull', 'dull']},
            index=MultiIndex.from_arrays([['bar', 'bar', 'foo', 'foo'],
                                          ['one', 'two', 'one', 'two']],
                                         names=['A', 'B']))
        assert_frame_equal(result, expected)


def test_nth_empty():
    # GH 16064
    df = DataFrame(index=[0], columns=['a', 'b', 'c'])
    result = df.groupby('a').nth(10)
    expected = DataFrame(index=Index([], name='a'), columns=['b', 'c'])
    assert_frame_equal(result, expected)

    result = df.groupby(['a', 'b']).nth(10)
    expected = DataFrame(index=MultiIndex([[], []], [[], []],
                                          names=['a', 'b']),
                         columns=['c'])
    assert_frame_equal(result, expected)