在 groupby 之后来自 nunique 的奇怪 output

Question

I am intrigued by the behavior of the following code:

import pandas as pd
df = pd.DataFrame({'Name':['A','A','B','B','B'],
                   'Date':['2020-01-01','2020-01-02','2020-01-01','2020-01-02','2020-01-03']})
df['Date'] = pd.to_datetime(df['Date'],infer_datetime_format=True)
df['Data_Points'] = df.groupby(['Name'])['Date'].transform('nunique')
print(df)

Which outputs:

  Name       Date                   Data_Points
0    A 2020-01-01 1970-01-01 00:00:00.000000002
1    A 2020-01-02 1970-01-01 00:00:00.000000002
2    B 2020-01-01 1970-01-01 00:00:00.000000003
3    B 2020-01-02 1970-01-01 00:00:00.000000003
4    B 2020-01-03 1970-01-01 00:00:00.000000003

The question comes as to why am I getting a datetime value after using transform('nunique') if the documentation for pandas.Series.nunique clear states:

Return number of unique elements in the object.

Returns: int

And pandas.DataFrame.transform does not mention anything about retaining the dtype of the aggregated column, only:

Call func on self producing a DataFrame with transformed values. Produced DataFrame will have same axis length as self.

So therefore, when combining both functions, why am I getting a datetime instead of the int as nunique() says? Does the aggregated dtype have precedence over the function being passed in the transform() method when defining the dtype of the transformed column? Is this the expected behavior?

Answer 1

I think it is bug, possible solution:

df['Data_Points'] = df.groupby(['Name'])['Date'].transform(pd.Series.nunique)
print(df)
  Name       Date  Data_Points
0    A 2020-01-01            2
1    A 2020-01-02            2
2    B 2020-01-01            3
3    B 2020-01-02            3
4    B 2020-01-03            3

Answer 2

IIUC, it is because the transformed result is inserted back as a datetime (the original) type. Add astype(int) solves it:

df.groupby('Name')["Date"].transform("nunique").astype(int)

Output:

0    2
1    2
2    3
3    3
4    3
Name: Date, dtype: int64