How to read/write to many text files

So in short, i have a directory that looks like

All the given text files look like

and I want to iterate through each file and input the text This is file #i at a certain line where i is the EXACT number of the file in its file name. I've watched countless videos on the os module, but still cant figure it out.

So just for reference,

is the goal im trying to reach for each file within the directory.

I would do it like this:

indirpath = 'path/to/indir/'
outdirpath = 'path/to/outdir/'
files = os.listdir(indirpath)
inp_pos = 2 # 0 indexed line number where you want to insert your text

for file in files:
    name = os.path.splitext(file)[0]     # get file name by removing extension
    inp_line = f'This is file #{name}'    # this is the line you have to input
    lines = open(os.path.join(indirpath, file)).read().strip().split('\n')
    lines.insert(min(inp_pos, len(lines)), inp_line) # insert inp_line at required position

    with open(os.path.join(outdirpath, file), 'w') as outfile:
        outfile.write('\n'.join(lines))

You can have indirpath and outdirpath as the same if your goal is to overwrite the original files.