How to efficiently append a new line to the starting of a large file?

Question

I want to append a new line in the starting of 2GB+ file. I tried following code but code OUT of MEMORY error.

myfile = open(tableTempFile, "r+")
myfile.read() # read everything in the file
myfile.seek(0) # rewind
myfile.write("WRITE IN THE FIRST LINE ")
myfile.close();

1. What is the way to write in a file file without getting the entire file in memory?
2. How to append a new line at starting of the file?

Answer 1

If you can afford having the entire file in memory at once:

first_line_update = "WRITE IN THE FIRST LINE \n"
with open(tableTempFile, 'r+') as f:
  lines = f.readlines()
  lines[0] = first_line_update
  f.writelines(lines)

otherwise:

from shutil import copy
from itertools import islice, chain 
# TODO: use a NamedTemporaryFile from the tempfile module
first_line_update = "WRITE IN THE FIRST LINE \n"
with open("inputfile", 'r') as infile, open("tmpfile", 'w+') as outfile:
  # replace the first line with the string provided:
  outfile.writelines(
    (line for line in chain((first_line_update,), islice(infile,1,None)))
  # if you don't want to replace the first line but to insert another line before
  # this simplifies to:
  #outfile.writelines(line for line in chain((first_line_update,), infile))
copy("tmpfile", "infile")
# TODO: remove temporary file