在带有日期的几列上具有多个条件的子集 pandas 数据框

Question

我有一个数据框如下：

  slot_id class        day   base_date
0        1     A     Monday  2019-01-21
1        2     B    Tuesday  2019-01-22
2        3     C  Wednesday  2019-01-23
3        4     C  Wednesday  2019-01-23
4        5     C   Thursday  2019-01-24

带有以下信息：

example.info()
<class 'pandas.core.frame.DataFrame'>
RangeIndex: 8 entries, 0 to 7
Data columns (total 4 columns):
slot_id      8 non-null int64
class        8 non-null object
day          8 non-null object
base_date    8 non-null object
dtypes: int64(1), object(3)
memory usage: 200.0+ bytes

我想获得带有class == "C" 的行，最小为base_date。我尝试了很多min 的组合都没有成功，比如这个：

example[(example['class']=="C") & min(example['base_date'])]

上面的代码有什么问题？我怎样才能得到满足上述条件的整行？ 完整的example 数据框：

{'slot_id': {0: 1, 1: 2, 2: 3, 3: 4, 4: 5, 5: 6, 6: 7, 7: 8}, 'class': {0: 'A', 1: 'B', 2: 'C', 3: 'C', 4: 'C', 5: 'C', 6: 'D', 7: 'E'}, 'day': {0: 'Monday', 1: 'Tuesday', 2: 'Wednesday', 3: 'Wednesday', 4: 'Thursday', 5: 'Tuesday', 6: 'Thursday', 7: 'Saturday'}, 'base_date': {0: datetime.date(2019, 1, 21), 1: datetime.date(2019, 1, 22), 2: datetime.date(2019, 1, 23), 3: datetime.date(2019, 1, 23), 4: datetime.date(2019, 1, 24), 5: datetime.date(2019, 1, 22), 6: datetime.date(2019, 1, 24), 7: datetime.date(2019, 1, 26)}}
example['base_date'] = pd.to_datetime(example['base_date'].astype(str), format='%d%m%Y')
example['base_date'] = example['base_date'].dt.date

Answer 1

你可以使用idxmin:

pd.to_datetime(df.loc[df['class'] == 'C', 'base_date']).idxmin()
# 2

df.iloc[pd.to_datetime(df.loc[df['class'] == 'C', 'base_date']).idxmin()]

slot_id               3
class                 C
day           Wednesday
base_date    2019-01-23
Name: 2, dtype: object

---

如果你需要重复这样做，一个更好的解决方案是将“base_date”预先转换为datetime类型：

df['base_date'] = pd.to_datetime(df['base_date'], errors='coerce')
df.iloc[df.loc[df['class'] == 'C', 'base_date'].idxmin()]

slot_id                        3
class                          C
day                    Wednesday
base_date    2019-01-23 00:00:00
Name: 2, dtype: object