Prepend a line to an existing file in Python

I need to add a single line to the first line of a text file and it looks like the only options available to me are more lines of code than I would expect from python. Something like this:

f = open('filename','r')
temp = f.read()
f.close()

f = open('filename', 'w')
f.write("#testfirstline")

f.write(temp)
f.close()

Is there no easier way? Additionally, I see this two-handle example more often than opening a single handle for reading and writing ('r+') - why is that?

Solution 1:

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.

Solution 2:

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.

Solution 3:

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

Solution 4:

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

Solution 5:

You can save one write call with this:

f.write('#testfirstline\n' + temp)

When using 'r+', you would have to rewind the file after reading and before writing.