Insert Text In Between File Lines In Python

I have a file that I am currently reading from using fo = open('file.txt', 'r') Then by doing file = open('newfile.txt', 'w') file.write(fo.read()) file.write('Hello at the end of

Solution 1:

First you should load the file using the open() method and then apply the .readlines() method, which splits on "\n" and returns a list, then you update the list of strings by inserting a new string in between the list, then simply write the contents of the list to the new file using the new_file.write("\n".join(updated_list))

NOTE: This method will only work for files which can be loaded in the memory.

with open("filename.txt", "r") as prev_file, open("new_filename.txt", "w") as new_file:
    prev_contents = prev_file.readlines()
    #Now prev_contents is a list of strings and you may add the new line to this list at any position
    prev_contents.insert(4, "\n This is a new line \n ")
    new_file.write("\n".join(prev_contents))

Solution 2:

readlines() is not recommended because it reads the whole file into memory. It is also not needed because you can iterate over the file directly.

The following code will insert Hello at line 2 at line 2

with open('file.txt', 'r') as f_in:
    with open('file2.txt','w') as f_out:
        for line_no, line in enumerate(f_in, 1):
            if line_no == 2:
                f_out.write('Hello at line 2\n')
            f_out.write(line)

Note the use of the with open('filename','w') as filevar idiom. This removes the need for an explicit close() because it closes the file automatically at the end of the block, and better, it does this even if there is an exception.

Solution 3:

For Large file

with open ("s.txt","r") as inp,open ("s1.txt","w") as ou:
    for a,d in enumerate(inp.readlines()):
        if a==2:
            ou.write("hi there\n")
        ou.write(d)