Dataframe not appending correctly

[scidam]

There are lots of questions here: What are B, D, F in line #6? what means 'xxx.hdf', why did you use key[0:-4], are all of your keys have length 5? Did you use pd.concat somewhere in your code? Should 'xxx.hdf'.format(key[0:-4]) be something like '{}.hdf'.format(key)?