Atmospheric Toolbox

KeyError in creating attribute

I was doing analysis of NO2 emission using Sentinel 5p following the training of RUS Copernicus ATM002. When I was creating an attribute function it works fine. But when I call it in a function it says KeyError. As I’m very beginner in python, I can not understand where is the problem. It would be nice if someone help me out. Here is my code and error msg.

attributes = {
        'time_coverage_start': xr.open_dataset(i).attrs['time_coverage_start']
    } for i in input_files_offl

#Print only first items in dictionary as example

def preprocess(ds):
        ds['time'] = pd.to_datetime(np.array([attributes[ds.attrs['source_product']]['time_coverage_start']])).values
        return ds

L3_march_20 = xr.open_mfdataset(filename_L3, combine='nested', concat_dim='time', preprocess=preprocess, chunks={'time':100})