How to use 'pandas groupby concatenate strings' in Python

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38def add_group_id(df, *groupby_cols, gid_colname='gid'):
39 groupby_cols = list(groupby_cols)
40 df_group = df.groupby(groupby_cols).apply(lambda g: pd.Series({
41 'group_length': g.shape[0]
42 })).reset_index()
43 df_group[gid_colname] = df_group.index
44 df_merge = pd.merge(df, df_group, how='outer', on=groupby_cols)
45 df_merge['group_length'] = df_merge['group_length'].fillna(-1)
46 df_merge[gid_colname] = df_merge[gid_colname].fillna(-1)
47 df_merge['group_length'] = df_merge['group_length'].astype(int)
48 df_merge[gid_colname] = df_merge[gid_colname].astype(int)
49 return df_merge

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40def test_deconcatenate_column_string_no_sep(dataframe):
41 with pytest.raises(ValueError):
42 df_orig = dataframe.concatenate_columns(
43 column_names=["a", "decorated-elephant"],
44 sep="-",
45 new_column_name="index",
46 )
47 df = df_orig.deconcatenate_column(
48 column_name="index", new_column_names=["A", "B"]
49 )

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