change first line of a file in python

shutil.copyfileobj() should be much faster than running line-by-line. Note from the docs:

Note that if the current file position of the [from_file] object is not 0, only the contents from the current file position to the end of the file will be copied.

Thus:

from_file.readline() # and discard
to_file.write(replacement_line)
shutil.copyfileobj(from_file, to_file)

An alternate solution that does not require iterating over the lines that are not of interest.

def replace_first_line( src_filename, target_filename, replacement_line):
    f = open(src_filename)
    first_line, remainder = f.readline(), f.read()
    t = open(target_filename,"w")
    t.write(replacement_line + "\n")
    t.write(remainder)
    t.close()

If you want to modify the top line of a file and save it under a new file name, it is not possible to simply modify the first line without iterating over the entire file. On the bright side, as long as you are not printing to the terminal, modifying the first line of a file is VERY, VERY fast even on vasy large files.

Assuming you are working with text-based files (not binary,) this should fit your needs and perform well enough for most applications.

import os
newline = os.linesep # Defines the newline based on your OS.

source_fp = open('source-filename', 'r')
target_fp = open('target-filename', 'w')
first_row = True
for row in source_fp:
    if first_row:
        row = 'the first row now says this.'
        first_row = False
    target_fp.write(row + newline)