Answer by pwuertz for Check to see if file exists is failing

This is a slightly modified version of Lattywares solution. Since it is not possible to append data to the beginning of a file, the whole content is read and the file is written anew including your header. By opening the file in read/write mode we can do both operations with the same file handler without releasing it. This should provide some protection against race conditions.

try:    with open(filename, 'r+') as f:        data = f.read()        f.seek(0,0)        f.write(header)        f.write(data)        #f.truncate() is not needed here as the file will always grow    print("Header added to", filename)except IOError:    print("Sorry, could not open file for reading/writing")