Can you please provide a *simple* and *complete* demonstration, including the *full* traceback? As given, we cannot run the supplied code and don't know what the contents of the csv file are supposed to be.

See here for more detail: http://www.sscce.org/

But my *guess*, on reading this, is that the line 

    header = fin.readline()

causes the difference. In the first sample, you skip past the first line of the csv file, and so when the dictreader reads the rest of the file, it never sees the first row. But in the second example, it does see the first row.

What happens if you change the first example to this?

        header = fin.readline()
        print('Found ' + header)
        fin.seek(0)
        reader = csv.DictReader(fin)

Does that solve your problem?

Likewise, in the second case, if you change it to this:

    open_f = open(filename, 'r', encoding='utf-8')
    __ = open_f.readline()
    read_it = csv.DictReader(open_f)


what happens?