pandas DataFrame 根据某列的缺失值，新增一列，表示该列的值是否缺失

>>> import pandas as pd
>>> import numpy as np
>>> df = pd.DataFrame({"A":[1,2,3],"B":[2,3,np.nan]})
>>> df
	A	B
0	1	2.0
1	2	3.0
2	3	NaN
>>> df.B.isna()  # B列的值为nan
0    False
1    False
2     True
Name: B, dtype: bool
>>> df.B.notna() # B列的值不为nan
0     True
1     True
2    False
Name: B, dtype: bool
>>> df["B_isNan"] = df["B"].notna().astype('int') # 新增一列，对应B是否缺失
>>> df
    A	B	B_isNan
0	1	2.0	1
1	2	3.0	1
2	3	NaN	0