Prepend a line to an existing file in Python

Python makes a lot of things easy and contains libraries and wrappers for a lot of common operations, but the goal is not to hide fundamental truths.

The fundamental truth you are encountering here is that you generally can't prepend data to an existing flat structure without rewriting the entire structure. This is true regardless of language.

There are ways to save a filehandle or make your code less readable, many of which are provided in other answers, but none change the fundamental operation: You must read in the existing file, then write out the data you want to prepend, followed by the existing data you read in.

By all means save yourself the filehandle, but don't go looking to pack this operation into as few lines of code as possible. In fact, never go looking for the fewest lines of code -- that's obfuscation, not programming.

with open("file", "r+") as f: s = f.read(); f.seek(0); f.write("prepend\n" + s)

Other approach:

with open("infile") as f1:
    with open("outfile", "w") as f2:
        f2.write("#test firstline")
        for line in f1:
            f2.write(line)

or a one liner:

open("outfile", "w").write("#test firstline\n" + open("infile").read())

Thanks for the opportunity to think about this problem :)

Cheers

I would stick with separate reads and writes, but we certainly can express each more concisely:

Python2:

with file('filename', 'r') as original: data = original.read()
with file('filename', 'w') as modified: modified.write("new first line\n" + data)

Python3:

with open('filename', 'r') as original: data = original.read()
with open('filename', 'w') as modified: modified.write("new first line\n" + data)

Note: file() function is not available in python3.