Python replace a line of file by indecie

Question

so basically I'm trying to figure out the best way to open a file and say replace the first line of the file with something else. Unfortunately, everything I come across uses .replace which only works if you know the string you want to replace. What is the best method if I know what line it appears on in the document and just want to replace that line with something else?

Code I have so far:

files = os.listdir(root.dirname)
    files = [s for s in files if ".xml" in s]
    print (files)
    for x in range(len(files)):
        with open(os.path.join(root.dirname,files[x]),"r+") as f:
            old = f.read()
            f.seek(0)

Answer 1

Ok so I got it working using this:

def replace_in_file(filename, idx, new_line):
    f = open(filename, "r")
    lines = f.readlines()
    f.close()
    lines[idx] = new_line + "\n"
    f = open(filename, "w")
    for line in lines:
        write=line
        f.write(write)
    f.close()
files = os.listdir(root.dirname)
    files = [s for s in files if ".xml" in s]
    for x in range(len(files)):
        #with open(os.path.join(root.dirname,files[x]),"r+") as f:
            file = os.path.join(root.dirname,files[x])
            print("Fixing "+file)
            replace = '<?xml-stylesheet type="text/xsl" href="'+root.filename+'"?>'
            replace_in_file(file, 0, replace)
            with open(os.path.join(root.dirname,files[x]),"r+") as f:
                old = f.read()
                f.close
            with open(os.path.join(root.dirname,files[x]),"w") as f:
                f.write('<?xml version="1.0" encoding="UTF-8"?>\n'+old)
                f.close

Answer 2

You can:

# Open a file.
f = open("file.txt")
# Read all the lines.
lines = f.readlines()
# Replace the first.
lines[0] = "replacing text"
# Close.
f.close()
# Open for writting.
f = open("file.txt", "w")
# Write
for line in lines:
    f.write(line)
# Close
f.close()

Answer 3

To replace a particular line of a file that depends how you know which line.

If you know the number of the line:

def replace_in_file(filename, idx, new_line):
    f = open(filename, "r")
    lines = f.readlines()
    f.close()
    lines[idx] = new_line + "\n"
    f = open("file", "w")
    f.write("".join(lines))
    f.close()

replace_in_file("file", 0, "new line content")

You can use the function replace_in_file providing the file name filename, the index idx of the line to change and the new content new_line you want.