change first line of a file in python

Question

I only need to read the first line of a huge file and change it.

Is there a trick to only change the first line of a file and save it as another file using Python? All my code is done in Python and would help me to keep consistency.

The idea is to not have to read and then write the whole file.

Answer 1

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)