replace nan values by mean group by date.year, date.month

[wissam1974]

Hi for all

i have read a CSV file with tow series columns as follow:
Dateobs TMIN
2006-01-01 NAN
2006-01-02 12.3
2006-01-03 11.3
..
2006-02-01 15.2
2006-02-02 Nan
2006-03-03 11.3
..
2016-04-06 15.8
2016-04-07 11.6
2016-04-08 Nan
..
etc..

Error:
my question is how to replace NaN values for the daily TMIN column by mean group by (year month)

the table shown above was issued when i execute this pseudo code

    data_pd = pd.read_csv('beirut.csv')

    # overwriting data after changing format
    data_pd["Dateobs"] = pd.to_datetime(data_pd["Dateobs"])
    Dateobs_typedatetime64=  data_pd["Dateobs"]
    print(Dateobs_typedatetime64)

    TMIN_typeFloat64=  data_pd["TMIN"]
    print(TMIN_typeFloat64)

any help or suggestions to overpass this complexity will be appreciate.
thank you