era_retrieval.py

import pandas as pd
import numpy as np
import datetime
import ecmwfapi
import xarray as xr
from collections import OrderedDict

dates = pd.Series([d.strftime("%Y-%m-%d") for d in pd.date_range("1979-01-01", "2016-12-31", freq="1D")], 
                  index=pd.date_range("1979-01-01", "2016-12-31", freq="1D"))

periods = [dates[str(year)+"-"+str(month)][0] + "/to/" + dates[str(year)+"-"+str(month)][-1] 
           for year in range(1979, 2017) for month in range(1, 13)]

server = ecmwfapi.ECMWFDataServer()

for period in periods[:1]:
    
    filename = period[:4]+period[5:7]+"_tmin.nc"
    
    data_dict = {"class": "ei", 
                 "dataset": "interim", 
                 "stream": "oper", 
                 "expver": "1", 
                 "date": period,                # month i need 
                 "type": "fc", 
                 "levtype": "sfc", 
                 "param": "201.128",             # 'Maximum temperature at 2 metres since previous post-processing' and 'Minimum temperature at 2 metres since previous post-processing' 
                 "step": "3/6/9/12",             # 4 steps per forecast 
                 "time": "00:00:00/12:00:00",    # 2 forecasts per day 
                 "grid": "0.125/0.125", 
                 "format": "netcdf",
                 "target": filename,             # change this to your output file name
}
    
    server.retrieve(data_dict)