Python code to batch convert multiple non-csv files to csv files in a given directory

Question

I am trying to batch convert anything other than a csv in a given directory to csv files(In this case, dat, txt files with the same filename)

import pandas as pd
import glob, os
parent_dir = 'c/path'
for dat_file in glob.glob(os.path.join(parent_dir, '*.dat')):
    #print (dat_file)
    fln = print(os.path.basename(dat_file))
    #print(fln)
    df = pd.read_csv(fln + ".dat", sep='|', header=None)
    hdr = ['a', 'b', 'c', 'd']
    df.to_csv(fln + ".csv", header=hdr, index=False)

for txt_file in glob.glob(os.path.join(parent_dir, '*.txt')):
        #print (txt_file)
        fln = print(os.path.basename(txt_file))
        #print(fln)
        df = pd.read_csv(fln + ".txt", sep='', header=None)
        hdr = ['a', 'b', 'c', 'd']
        df.to_csv(fln + ".csv", header=hdr, index=False)

But when I am trying to execute, it says, although the syntax seems correct.

TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'