manyi/venv/Lib/site-packages/pandas/tests/groupby/test_grouping.py

# -*- coding: utf-8 -*-

""" test where we are determining what we are grouping, or getting groups """

import numpy as np
import pytest

from pandas.compat import long, lrange

import pandas as pd
from pandas import (
    CategoricalIndex, DataFrame, Index, MultiIndex, Series, Timestamp, compat,
    date_range)
from pandas.core.groupby.grouper import Grouping
import pandas.util.testing as tm
from pandas.util.testing import (
    assert_almost_equal, assert_frame_equal, assert_panel_equal,
    assert_series_equal)

# selection
# --------------------------------


class TestSelection(object):

    def test_select_bad_cols(self):
        df = DataFrame([[1, 2]], columns=['A', 'B'])
        g = df.groupby('A')
        with pytest.raises(KeyError, match='"Columns not found: \'C\'"'):
            g[['C']]

        with pytest.raises(KeyError, match='^[^A]+$'):
            # A should not be referenced as a bad column...
            # will have to rethink regex if you change message!
            g[['A', 'C']]

    def test_groupby_duplicated_column_errormsg(self):
        # GH7511
        df = DataFrame(columns=['A', 'B', 'A', 'C'],
                       data=[range(4), range(2, 6), range(0, 8, 2)])

        msg = "Grouper for 'A' not 1-dimensional"
        with pytest.raises(ValueError, match=msg):
            df.groupby('A')
        with pytest.raises(ValueError, match=msg):
            df.groupby(['A', 'B'])

        grouped = df.groupby('B')
        c = grouped.count()
        assert c.columns.nlevels == 1
        assert c.columns.size == 3

    def test_column_select_via_attr(self, df):
        result = df.groupby('A').C.sum()
        expected = df.groupby('A')['C'].sum()
        assert_series_equal(result, expected)

        df['mean'] = 1.5
        result = df.groupby('A').mean()
        expected = df.groupby('A').agg(np.mean)
        assert_frame_equal(result, expected)

    def test_getitem_list_of_columns(self):
        df = DataFrame(
            {'A': ['foo', 'bar', 'foo', 'bar', 'foo', 'bar', 'foo', 'foo'],
             'B': ['one', 'one', 'two', 'three', 'two', 'two', 'one', 'three'],
             'C': np.random.randn(8),
             'D': np.random.randn(8),
             'E': np.random.randn(8)})

        result = df.groupby('A')[['C', 'D']].mean()
        result2 = df.groupby('A')['C', 'D'].mean()
        result3 = df.groupby('A')[df.columns[2:4]].mean()

        expected = df.loc[:, ['A', 'C', 'D']].groupby('A').mean()

        assert_frame_equal(result, expected)
        assert_frame_equal(result2, expected)
        assert_frame_equal(result3, expected)

    def test_getitem_numeric_column_names(self):
        # GH #13731
        df = DataFrame({0: list('abcd') * 2,
                        2: np.random.randn(8),
                        4: np.random.randn(8),
                        6: np.random.randn(8)})
        result = df.groupby(0)[df.columns[1:3]].mean()
        result2 = df.groupby(0)[2, 4].mean()
        result3 = df.groupby(0)[[2, 4]].mean()

        expected = df.loc[:, [0, 2, 4]].groupby(0).mean()

        assert_frame_equal(result, expected)
        assert_frame_equal(result2, expected)
        assert_frame_equal(result3, expected)


# grouping
# --------------------------------

class TestGrouping():

    def test_grouper_index_types(self):
        # related GH5375
        # groupby misbehaving when using a Floatlike index
        df = DataFrame(np.arange(10).reshape(5, 2), columns=list('AB'))
        for index in [tm.makeFloatIndex, tm.makeStringIndex,
                      tm.makeUnicodeIndex, tm.makeIntIndex, tm.makeDateIndex,
                      tm.makePeriodIndex]:

            df.index = index(len(df))
            df.groupby(list('abcde')).apply(lambda x: x)

            df.index = list(reversed(df.index.tolist()))
            df.groupby(list('abcde')).apply(lambda x: x)

    def test_grouper_multilevel_freq(self):

        # GH 7885
        # with level and freq specified in a pd.Grouper
        from datetime import date, timedelta
        d0 = date.today() - timedelta(days=14)
        dates = date_range(d0, date.today())
        date_index = pd.MultiIndex.from_product(
            [dates, dates], names=['foo', 'bar'])
        df = pd.DataFrame(np.random.randint(0, 100, 225), index=date_index)

        # Check string level
        expected = df.reset_index().groupby([pd.Grouper(
            key='foo', freq='W'), pd.Grouper(key='bar', freq='W')]).sum()
        # reset index changes columns dtype to object
        expected.columns = pd.Index([0], dtype='int64')

        result = df.groupby([pd.Grouper(level='foo', freq='W'), pd.Grouper(
            level='bar', freq='W')]).sum()
        assert_frame_equal(result, expected)

        # Check integer level
        result = df.groupby([pd.Grouper(level=0, freq='W'), pd.Grouper(
            level=1, freq='W')]).sum()
        assert_frame_equal(result, expected)

    def test_grouper_creation_bug(self):

        # GH 8795
        df = DataFrame({'A': [0, 0, 1, 1, 2, 2], 'B': [1, 2, 3, 4, 5, 6]})
        g = df.groupby('A')
        expected = g.sum()

        g = df.groupby(pd.Grouper(key='A'))
        result = g.sum()
        assert_frame_equal(result, expected)

        result = g.apply(lambda x: x.sum())
        assert_frame_equal(result, expected)

        g = df.groupby(pd.Grouper(key='A', axis=0))
        result = g.sum()
        assert_frame_equal(result, expected)

        # GH14334
        # pd.Grouper(key=...) may be passed in a list
        df = DataFrame({'A': [0, 0, 0, 1, 1, 1],
                        'B': [1, 1, 2, 2, 3, 3],
                        'C': [1, 2, 3, 4, 5, 6]})
        # Group by single column
        expected = df.groupby('A').sum()
        g = df.groupby([pd.Grouper(key='A')])
        result = g.sum()
        assert_frame_equal(result, expected)

        # Group by two columns
        # using a combination of strings and Grouper objects
        expected = df.groupby(['A', 'B']).sum()

        # Group with two Grouper objects
        g = df.groupby([pd.Grouper(key='A'), pd.Grouper(key='B')])
        result = g.sum()
        assert_frame_equal(result, expected)

        # Group with a string and a Grouper object
        g = df.groupby(['A', pd.Grouper(key='B')])
        result = g.sum()
        assert_frame_equal(result, expected)

        # Group with a Grouper object and a string
        g = df.groupby([pd.Grouper(key='A'), 'B'])
        result = g.sum()
        assert_frame_equal(result, expected)

        # GH8866
        s = Series(np.arange(8, dtype='int64'),
                   index=pd.MultiIndex.from_product(
                       [list('ab'), range(2),
                        date_range('20130101', periods=2)],
                       names=['one', 'two', 'three']))
        result = s.groupby(pd.Grouper(level='three', freq='M')).sum()
        expected = Series([28], index=Index(
            [Timestamp('2013-01-31')], freq='M', name='three'))
        assert_series_equal(result, expected)

        # just specifying a level breaks
        result = s.groupby(pd.Grouper(level='one')).sum()
        expected = s.groupby(level='one').sum()
        assert_series_equal(result, expected)

    def test_grouper_column_and_index(self):
        # GH 14327

        # Grouping a multi-index frame by a column and an index level should
        # be equivalent to resetting the index and grouping by two columns
        idx = pd.MultiIndex.from_tuples([('a', 1), ('a', 2), ('a', 3),
                                         ('b', 1), ('b', 2), ('b', 3)])
        idx.names = ['outer', 'inner']
        df_multi = pd.DataFrame({"A": np.arange(6),
                                 'B': ['one', 'one', 'two',
                                       'two', 'one', 'one']},
                                index=idx)
        result = df_multi.groupby(['B', pd.Grouper(level='inner')]).mean()
        expected = df_multi.reset_index().groupby(['B', 'inner']).mean()
        assert_frame_equal(result, expected)

        # Test the reverse grouping order
        result = df_multi.groupby([pd.Grouper(level='inner'), 'B']).mean()
        expected = df_multi.reset_index().groupby(['inner', 'B']).mean()
        assert_frame_equal(result, expected)

        # Grouping a single-index frame by a column and the index should
        # be equivalent to resetting the index and grouping by two columns
        df_single = df_multi.reset_index('outer')
        result = df_single.groupby(['B', pd.Grouper(level='inner')]).mean()
        expected = df_single.reset_index().groupby(['B', 'inner']).mean()
        assert_frame_equal(result, expected)

        # Test the reverse grouping order
        result = df_single.groupby([pd.Grouper(level='inner'), 'B']).mean()
        expected = df_single.reset_index().groupby(['inner', 'B']).mean()
        assert_frame_equal(result, expected)

    def test_groupby_levels_and_columns(self):
        # GH9344, GH9049
        idx_names = ['x', 'y']
        idx = pd.MultiIndex.from_tuples(
            [(1, 1), (1, 2), (3, 4), (5, 6)], names=idx_names)
        df = pd.DataFrame(np.arange(12).reshape(-1, 3), index=idx)

        by_levels = df.groupby(level=idx_names).mean()
        # reset_index changes columns dtype to object
        by_columns = df.reset_index().groupby(idx_names).mean()

        tm.assert_frame_equal(by_levels, by_columns, check_column_type=False)

        by_columns.columns = pd.Index(by_columns.columns, dtype=np.int64)
        tm.assert_frame_equal(by_levels, by_columns)

    def test_groupby_categorical_index_and_columns(self, observed):
        # GH18432
        columns = ['A', 'B', 'A', 'B']
        categories = ['B', 'A']
        data = np.ones((5, 4), int)
        cat_columns = CategoricalIndex(columns,
                                       categories=categories,
                                       ordered=True)
        df = DataFrame(data=data, columns=cat_columns)
        result = df.groupby(axis=1, level=0, observed=observed).sum()
        expected_data = 2 * np.ones((5, 2), int)

        if observed:
            # if we are not-observed we undergo a reindex
            # so need to adjust the output as our expected sets us up
            # to be non-observed
            expected_columns = CategoricalIndex(['A', 'B'],
                                                categories=categories,
                                                ordered=True)
        else:
            expected_columns = CategoricalIndex(categories,
                                                categories=categories,
                                                ordered=True)
        expected = DataFrame(data=expected_data, columns=expected_columns)
        assert_frame_equal(result, expected)

        # test transposed version
        df = DataFrame(data.T, index=cat_columns)
        result = df.groupby(axis=0, level=0, observed=observed).sum()
        expected = DataFrame(data=expected_data.T, index=expected_columns)
        assert_frame_equal(result, expected)

    def test_grouper_getting_correct_binner(self):

        # GH 10063
        # using a non-time-based grouper and a time-based grouper
        # and specifying levels
        df = DataFrame({'A': 1}, index=pd.MultiIndex.from_product(
            [list('ab'), date_range('20130101', periods=80)], names=['one',
                                                                     'two']))
        result = df.groupby([pd.Grouper(level='one'), pd.Grouper(
            level='two', freq='M')]).sum()
        expected = DataFrame({'A': [31, 28, 21, 31, 28, 21]},
                             index=MultiIndex.from_product(
                                 [list('ab'),
                                  date_range('20130101', freq='M', periods=3)],
                                 names=['one', 'two']))
        assert_frame_equal(result, expected)

    def test_grouper_iter(self, df):
        assert sorted(df.groupby('A').grouper) == ['bar', 'foo']

    def test_empty_groups(self, df):
        # see gh-1048
        with pytest.raises(ValueError, match="No group keys passed!"):
            df.groupby([])

    def test_groupby_grouper(self, df):
        grouped = df.groupby('A')

        result = df.groupby(grouped.grouper).mean()
        expected = grouped.mean()
        tm.assert_frame_equal(result, expected)

    def test_groupby_dict_mapping(self):
        # GH #679
        from pandas import Series
        s = Series({'T1': 5})
        result = s.groupby({'T1': 'T2'}).agg(sum)
        expected = s.groupby(['T2']).agg(sum)
        assert_series_equal(result, expected)

        s = Series([1., 2., 3., 4.], index=list('abcd'))
        mapping = {'a': 0, 'b': 0, 'c': 1, 'd': 1}

        result = s.groupby(mapping).mean()
        result2 = s.groupby(mapping).agg(np.mean)
        expected = s.groupby([0, 0, 1, 1]).mean()
        expected2 = s.groupby([0, 0, 1, 1]).mean()
        assert_series_equal(result, expected)
        assert_series_equal(result, result2)
        assert_series_equal(result, expected2)

    def test_groupby_grouper_f_sanity_checked(self):
        dates = date_range('01-Jan-2013', periods=12, freq='MS')
        ts = Series(np.random.randn(12), index=dates)

        # GH3035
        # index.map is used to apply grouper to the index
        # if it fails on the elements, map tries it on the entire index as
        # a sequence. That can yield invalid results that cause trouble
        # down the line.
        # the surprise comes from using key[0:6] rather then str(key)[0:6]
        # when the elements are Timestamp.
        # the result is Index[0:6], very confusing.

        msg = r"Grouper result violates len\(labels\) == len\(data\)"
        with pytest.raises(AssertionError, match=msg):
            ts.groupby(lambda key: key[0:6])

    def test_grouping_error_on_multidim_input(self, df):
        msg = ("Grouper for '<class 'pandas.core.frame.DataFrame'>'"
               " not 1-dimensional")
        with pytest.raises(ValueError, match=msg):
            Grouping(df.index, df[['A', 'A']])

    def test_multiindex_passthru(self):

        # GH 7997
        # regression from 0.14.1
        df = pd.DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
        df.columns = pd.MultiIndex.from_tuples([(0, 1), (1, 1), (2, 1)])

        result = df.groupby(axis=1, level=[0, 1]).first()
        assert_frame_equal(result, df)

    def test_multiindex_negative_level(self, mframe):
        # GH 13901
        result = mframe.groupby(level=-1).sum()
        expected = mframe.groupby(level='second').sum()
        assert_frame_equal(result, expected)

        result = mframe.groupby(level=-2).sum()
        expected = mframe.groupby(level='first').sum()
        assert_frame_equal(result, expected)

        result = mframe.groupby(level=[-2, -1]).sum()
        expected = mframe
        assert_frame_equal(result, expected)

        result = mframe.groupby(level=[-1, 'first']).sum()
        expected = mframe.groupby(level=['second', 'first']).sum()
        assert_frame_equal(result, expected)

    def test_multifunc_select_col_integer_cols(self, df):
        df.columns = np.arange(len(df.columns))

        # it works!
        df.groupby(1, as_index=False)[2].agg({'Q': np.mean})

    def test_multiindex_columns_empty_level(self):
        lst = [['count', 'values'], ['to filter', '']]
        midx = MultiIndex.from_tuples(lst)

        df = DataFrame([[long(1), 'A']], columns=midx)

        grouped = df.groupby('to filter').groups
        assert grouped['A'] == [0]

        grouped = df.groupby([('to filter', '')]).groups
        assert grouped['A'] == [0]

        df = DataFrame([[long(1), 'A'], [long(2), 'B']], columns=midx)

        expected = df.groupby('to filter').groups
        result = df.groupby([('to filter', '')]).groups
        assert result == expected

        df = DataFrame([[long(1), 'A'], [long(2), 'A']], columns=midx)

        expected = df.groupby('to filter').groups
        result = df.groupby([('to filter', '')]).groups
        tm.assert_dict_equal(result, expected)

    def test_groupby_multiindex_tuple(self):
        # GH 17979
        df = pd.DataFrame([[1, 2, 3, 4], [3, 4, 5, 6], [1, 4, 2, 3]],
                          columns=pd.MultiIndex.from_arrays(
                              [['a', 'b', 'b', 'c'],
                               [1, 1, 2, 2]]))
        expected = df.groupby([('b', 1)]).groups
        result = df.groupby(('b', 1)).groups
        tm.assert_dict_equal(expected, result)

        df2 = pd.DataFrame(df.values,
                           columns=pd.MultiIndex.from_arrays(
                               [['a', 'b', 'b', 'c'],
                                ['d', 'd', 'e', 'e']]))
        expected = df2.groupby([('b', 'd')]).groups
        result = df.groupby(('b', 1)).groups
        tm.assert_dict_equal(expected, result)

        df3 = pd.DataFrame(df.values,
                           columns=[('a', 'd'), ('b', 'd'), ('b', 'e'), 'c'])
        expected = df3.groupby([('b', 'd')]).groups
        result = df.groupby(('b', 1)).groups
        tm.assert_dict_equal(expected, result)

    @pytest.mark.parametrize('sort', [True, False])
    def test_groupby_level(self, sort, mframe, df):
        # GH 17537
        frame = mframe
        deleveled = frame.reset_index()

        result0 = frame.groupby(level=0, sort=sort).sum()
        result1 = frame.groupby(level=1, sort=sort).sum()

        expected0 = frame.groupby(deleveled['first'].values, sort=sort).sum()
        expected1 = frame.groupby(deleveled['second'].values, sort=sort).sum()

        expected0.index.name = 'first'
        expected1.index.name = 'second'

        assert result0.index.name == 'first'
        assert result1.index.name == 'second'

        assert_frame_equal(result0, expected0)
        assert_frame_equal(result1, expected1)
        assert result0.index.name == frame.index.names[0]
        assert result1.index.name == frame.index.names[1]

        # groupby level name
        result0 = frame.groupby(level='first', sort=sort).sum()
        result1 = frame.groupby(level='second', sort=sort).sum()
        assert_frame_equal(result0, expected0)
        assert_frame_equal(result1, expected1)

        # axis=1

        result0 = frame.T.groupby(level=0, axis=1, sort=sort).sum()
        result1 = frame.T.groupby(level=1, axis=1, sort=sort).sum()
        assert_frame_equal(result0, expected0.T)
        assert_frame_equal(result1, expected1.T)

        # raise exception for non-MultiIndex
        msg = "level > 0 or level < -1 only valid with MultiIndex"
        with pytest.raises(ValueError, match=msg):
            df.groupby(level=1)

    def test_groupby_level_index_names(self):
        # GH4014 this used to raise ValueError since 'exp'>1 (in py2)
        df = DataFrame({'exp': ['A'] * 3 + ['B'] * 3,
                        'var1': lrange(6), }).set_index('exp')
        df.groupby(level='exp')
        msg = "level name foo is not the name of the index"
        with pytest.raises(ValueError, match=msg):
            df.groupby(level='foo')

    @pytest.mark.parametrize('sort', [True, False])
    def test_groupby_level_with_nas(self, sort):
        # GH 17537
        index = MultiIndex(levels=[[1, 0], [0, 1, 2, 3]],
                           codes=[[1, 1, 1, 1, 0, 0, 0, 0], [0, 1, 2, 3, 0, 1,
                                                             2, 3]])

        # factorizing doesn't confuse things
        s = Series(np.arange(8.), index=index)
        result = s.groupby(level=0, sort=sort).sum()
        expected = Series([6., 22.], index=[0, 1])
        assert_series_equal(result, expected)

        index = MultiIndex(levels=[[1, 0], [0, 1, 2, 3]],
                           codes=[[1, 1, 1, 1, -1, 0, 0, 0], [0, 1, 2, 3, 0,
                                                              1, 2, 3]])

        # factorizing doesn't confuse things
        s = Series(np.arange(8.), index=index)
        result = s.groupby(level=0, sort=sort).sum()
        expected = Series([6., 18.], index=[0.0, 1.0])
        assert_series_equal(result, expected)

    def test_groupby_args(self, mframe):
        # PR8618 and issue 8015
        frame = mframe

        msg = "You have to supply one of 'by' and 'level'"
        with pytest.raises(TypeError, match=msg):
            frame.groupby()

        msg = "You have to supply one of 'by' and 'level'"
        with pytest.raises(TypeError, match=msg):
            frame.groupby(by=None, level=None)

    @pytest.mark.parametrize('sort,labels', [
        [True, [2, 2, 2, 0, 0, 1, 1, 3, 3, 3]],
        [False, [0, 0, 0, 1, 1, 2, 2, 3, 3, 3]]
    ])
    def test_level_preserve_order(self, sort, labels, mframe):
        # GH 17537
        grouped = mframe.groupby(level=0, sort=sort)
        exp_labels = np.array(labels, np.intp)
        assert_almost_equal(grouped.grouper.labels[0], exp_labels)

    def test_grouping_labels(self, mframe):
        grouped = mframe.groupby(mframe.index.get_level_values(0))
        exp_labels = np.array([2, 2, 2, 0, 0, 1, 1, 3, 3, 3], dtype=np.intp)
        assert_almost_equal(grouped.grouper.labels[0], exp_labels)

    def test_list_grouper_with_nat(self):
        # GH 14715
        df = pd.DataFrame({'date': pd.date_range('1/1/2011',
                                                 periods=365, freq='D')})
        df.iloc[-1] = pd.NaT
        grouper = pd.Grouper(key='date', freq='AS')

        # Grouper in a list grouping
        result = df.groupby([grouper])
        expected = {pd.Timestamp('2011-01-01'): pd.Index(list(range(364)))}
        tm.assert_dict_equal(result.groups, expected)

        # Test case without a list
        result = df.groupby(grouper)
        expected = {pd.Timestamp('2011-01-01'): 365}
        tm.assert_dict_equal(result.groups, expected)


# get_group
# --------------------------------

class TestGetGroup():

    @pytest.mark.filterwarnings("ignore:\\nPanel:FutureWarning")
    def test_get_group(self):
        wp = tm.makePanel()
        grouped = wp.groupby(lambda x: x.month, axis='major')

        gp = grouped.get_group(1)
        expected = wp.reindex(
            major=[x for x in wp.major_axis if x.month == 1])
        assert_panel_equal(gp, expected)

        # GH 5267
        # be datelike friendly
        df = DataFrame({'DATE': pd.to_datetime(
            ['10-Oct-2013', '10-Oct-2013', '10-Oct-2013', '11-Oct-2013',
             '11-Oct-2013', '11-Oct-2013']),
            'label': ['foo', 'foo', 'bar', 'foo', 'foo', 'bar'],
            'VAL': [1, 2, 3, 4, 5, 6]})

        g = df.groupby('DATE')
        key = list(g.groups)[0]
        result1 = g.get_group(key)
        result2 = g.get_group(Timestamp(key).to_pydatetime())
        result3 = g.get_group(str(Timestamp(key)))
        assert_frame_equal(result1, result2)
        assert_frame_equal(result1, result3)

        g = df.groupby(['DATE', 'label'])

        key = list(g.groups)[0]
        result1 = g.get_group(key)
        result2 = g.get_group((Timestamp(key[0]).to_pydatetime(), key[1]))
        result3 = g.get_group((str(Timestamp(key[0])), key[1]))
        assert_frame_equal(result1, result2)
        assert_frame_equal(result1, result3)

        # must pass a same-length tuple with multiple keys
        msg = "must supply a tuple to get_group with multiple grouping keys"
        with pytest.raises(ValueError, match=msg):
            g.get_group('foo')
        with pytest.raises(ValueError, match=msg):
            g.get_group(('foo'))
        msg = ("must supply a same-length tuple to get_group with multiple"
               " grouping keys")
        with pytest.raises(ValueError, match=msg):
            g.get_group(('foo', 'bar', 'baz'))

    def test_get_group_empty_bins(self, observed):

        d = pd.DataFrame([3, 1, 7, 6])
        bins = [0, 5, 10, 15]
        g = d.groupby(pd.cut(d[0], bins), observed=observed)

        # TODO: should prob allow a str of Interval work as well
        # IOW '(0, 5]'
        result = g.get_group(pd.Interval(0, 5))
        expected = DataFrame([3, 1], index=[0, 1])
        assert_frame_equal(result, expected)

        msg = r"Interval\(10, 15, closed='right'\)"
        with pytest.raises(KeyError, match=msg):
            g.get_group(pd.Interval(10, 15))

    def test_get_group_grouped_by_tuple(self):
        # GH 8121
        df = DataFrame([[(1, ), (1, 2), (1, ), (1, 2)]], index=['ids']).T
        gr = df.groupby('ids')
        expected = DataFrame({'ids': [(1, ), (1, )]}, index=[0, 2])
        result = gr.get_group((1, ))
        assert_frame_equal(result, expected)

        dt = pd.to_datetime(['2010-01-01', '2010-01-02', '2010-01-01',
                             '2010-01-02'])
        df = DataFrame({'ids': [(x, ) for x in dt]})
        gr = df.groupby('ids')
        result = gr.get_group(('2010-01-01', ))
        expected = DataFrame({'ids': [(dt[0], ), (dt[0], )]}, index=[0, 2])
        assert_frame_equal(result, expected)

    def test_groupby_with_empty(self):
        index = pd.DatetimeIndex(())
        data = ()
        series = pd.Series(data, index)
        grouper = pd.Grouper(freq='D')
        grouped = series.groupby(grouper)
        assert next(iter(grouped), None) is None

    def test_groupby_with_single_column(self):
        df = pd.DataFrame({'a': list('abssbab')})
        tm.assert_frame_equal(df.groupby('a').get_group('a'), df.iloc[[0, 5]])
        # GH 13530
        exp = pd.DataFrame([], index=pd.Index(['a', 'b', 's'], name='a'))
        tm.assert_frame_equal(df.groupby('a').count(), exp)
        tm.assert_frame_equal(df.groupby('a').sum(), exp)
        tm.assert_frame_equal(df.groupby('a').nth(1), exp)

    def test_gb_key_len_equal_axis_len(self):
            # GH16843
            # test ensures that index and column keys are recognized correctly
            # when number of keys equals axis length of groupby
            df = pd.DataFrame([['foo', 'bar', 'B', 1],
                               ['foo', 'bar', 'B', 2],
                               ['foo', 'baz', 'C', 3]],
                              columns=['first', 'second', 'third', 'one'])
            df = df.set_index(['first', 'second'])
            df = df.groupby(['first', 'second', 'third']).size()
            assert df.loc[('foo', 'bar', 'B')] == 2
            assert df.loc[('foo', 'baz', 'C')] == 1


# groups & iteration
# --------------------------------

class TestIteration():

    def test_groups(self, df):
        grouped = df.groupby(['A'])
        groups = grouped.groups
        assert groups is grouped.groups  # caching works

        for k, v in compat.iteritems(grouped.groups):
            assert (df.loc[v]['A'] == k).all()

        grouped = df.groupby(['A', 'B'])
        groups = grouped.groups
        assert groups is grouped.groups  # caching works

        for k, v in compat.iteritems(grouped.groups):
            assert (df.loc[v]['A'] == k[0]).all()
            assert (df.loc[v]['B'] == k[1]).all()

    def test_grouping_is_iterable(self, tsframe):
        # this code path isn't used anywhere else
        # not sure it's useful
        grouped = tsframe.groupby([lambda x: x.weekday(), lambda x: x.year])

        # test it works
        for g in grouped.grouper.groupings[0]:
            pass

    def test_multi_iter(self):
        s = Series(np.arange(6))
        k1 = np.array(['a', 'a', 'a', 'b', 'b', 'b'])
        k2 = np.array(['1', '2', '1', '2', '1', '2'])

        grouped = s.groupby([k1, k2])

        iterated = list(grouped)
        expected = [('a', '1', s[[0, 2]]), ('a', '2', s[[1]]),
                    ('b', '1', s[[4]]), ('b', '2', s[[3, 5]])]
        for i, ((one, two), three) in enumerate(iterated):
            e1, e2, e3 = expected[i]
            assert e1 == one
            assert e2 == two
            assert_series_equal(three, e3)

    def test_multi_iter_frame(self, three_group):
        k1 = np.array(['b', 'b', 'b', 'a', 'a', 'a'])
        k2 = np.array(['1', '2', '1', '2', '1', '2'])
        df = DataFrame({'v1': np.random.randn(6),
                        'v2': np.random.randn(6),
                        'k1': k1, 'k2': k2},
                       index=['one', 'two', 'three', 'four', 'five', 'six'])

        grouped = df.groupby(['k1', 'k2'])

        # things get sorted!
        iterated = list(grouped)
        idx = df.index
        expected = [('a', '1', df.loc[idx[[4]]]),
                    ('a', '2', df.loc[idx[[3, 5]]]),
                    ('b', '1', df.loc[idx[[0, 2]]]),
                    ('b', '2', df.loc[idx[[1]]])]
        for i, ((one, two), three) in enumerate(iterated):
            e1, e2, e3 = expected[i]
            assert e1 == one
            assert e2 == two
            assert_frame_equal(three, e3)

        # don't iterate through groups with no data
        df['k1'] = np.array(['b', 'b', 'b', 'a', 'a', 'a'])
        df['k2'] = np.array(['1', '1', '1', '2', '2', '2'])
        grouped = df.groupby(['k1', 'k2'])
        groups = {key: gp for key, gp in grouped}
        assert len(groups) == 2

        # axis = 1
        three_levels = three_group.groupby(['A', 'B', 'C']).mean()
        grouped = three_levels.T.groupby(axis=1, level=(1, 2))
        for key, group in grouped:
            pass

    @pytest.mark.filterwarnings("ignore:\\nPanel:FutureWarning")
    def test_multi_iter_panel(self):
        wp = tm.makePanel()
        grouped = wp.groupby([lambda x: x.month, lambda x: x.weekday()],
                             axis=1)

        for (month, wd), group in grouped:
            exp_axis = [x
                        for x in wp.major_axis
                        if x.month == month and x.weekday() == wd]
            expected = wp.reindex(major=exp_axis)
            assert_panel_equal(group, expected)

    def test_dictify(self, df):
        dict(iter(df.groupby('A')))
        dict(iter(df.groupby(['A', 'B'])))
        dict(iter(df['C'].groupby(df['A'])))
        dict(iter(df['C'].groupby([df['A'], df['B']])))
        dict(iter(df.groupby('A')['C']))
        dict(iter(df.groupby(['A', 'B'])['C']))

    def test_groupby_with_small_elem(self):
        # GH 8542
        # length=2
        df = pd.DataFrame({'event': ['start', 'start'],
                           'change': [1234, 5678]},
                          index=pd.DatetimeIndex(['2014-09-10', '2013-10-10']))
        grouped = df.groupby([pd.Grouper(freq='M'), 'event'])
        assert len(grouped.groups) == 2
        assert grouped.ngroups == 2
        assert (pd.Timestamp('2014-09-30'), 'start') in grouped.groups
        assert (pd.Timestamp('2013-10-31'), 'start') in grouped.groups

        res = grouped.get_group((pd.Timestamp('2014-09-30'), 'start'))
        tm.assert_frame_equal(res, df.iloc[[0], :])
        res = grouped.get_group((pd.Timestamp('2013-10-31'), 'start'))
        tm.assert_frame_equal(res, df.iloc[[1], :])

        df = pd.DataFrame({'event': ['start', 'start', 'start'],
                           'change': [1234, 5678, 9123]},
                          index=pd.DatetimeIndex(['2014-09-10', '2013-10-10',
                                                  '2014-09-15']))
        grouped = df.groupby([pd.Grouper(freq='M'), 'event'])
        assert len(grouped.groups) == 2
        assert grouped.ngroups == 2
        assert (pd.Timestamp('2014-09-30'), 'start') in grouped.groups
        assert (pd.Timestamp('2013-10-31'), 'start') in grouped.groups

        res = grouped.get_group((pd.Timestamp('2014-09-30'), 'start'))
        tm.assert_frame_equal(res, df.iloc[[0, 2], :])
        res = grouped.get_group((pd.Timestamp('2013-10-31'), 'start'))
        tm.assert_frame_equal(res, df.iloc[[1], :])

        # length=3
        df = pd.DataFrame({'event': ['start', 'start', 'start'],
                           'change': [1234, 5678, 9123]},
                          index=pd.DatetimeIndex(['2014-09-10', '2013-10-10',
                                                  '2014-08-05']))
        grouped = df.groupby([pd.Grouper(freq='M'), 'event'])
        assert len(grouped.groups) == 3
        assert grouped.ngroups == 3
        assert (pd.Timestamp('2014-09-30'), 'start') in grouped.groups
        assert (pd.Timestamp('2013-10-31'), 'start') in grouped.groups
        assert (pd.Timestamp('2014-08-31'), 'start') in grouped.groups

        res = grouped.get_group((pd.Timestamp('2014-09-30'), 'start'))
        tm.assert_frame_equal(res, df.iloc[[0], :])
        res = grouped.get_group((pd.Timestamp('2013-10-31'), 'start'))
        tm.assert_frame_equal(res, df.iloc[[1], :])
        res = grouped.get_group((pd.Timestamp('2014-08-31'), 'start'))
        tm.assert_frame_equal(res, df.iloc[[2], :])

    def test_grouping_string_repr(self):
        # GH 13394
        mi = MultiIndex.from_arrays([list("AAB"), list("aba")])
        df = DataFrame([[1, 2, 3]], columns=mi)
        gr = df.groupby(df[('A', 'a')])

        result = gr.grouper.groupings[0].__repr__()
        expected = "Grouping(('A', 'a'))"
        assert result == expected