举例：有df如下

>import numpy as np
>df = pd.DataFrame({"name":["apple", "pear", "pig", "dog", "cat"], "number_1":[1,np.nan,3,np.nan,5], "number_2":[np.nan, 2,3,4,5]})
>print(df)

    name  number_1  number_2
0  apple       1.0       NaN
1   pear       NaN       2.0
2    pig       3.0       3.0
3    dog       NaN       4.0
4    cat       5.0       5.0

要求：将number_1中的空值用number_2中的非空值进行填充。方法如下：

>df["number_1"].fillna(df["number_2"], inplace=True)
>print(df)

    name  number_1  number_2
0  apple       1.0       NaN
1   pear       2.0       2.0
2    pig       3.0       3.0
3    dog       4.0       4.0
4    cat       5.0       5.0