Insert comma after numbers

Question

In this code I'm trying to read multiple files in a folder, and insert a comma after each number. In the first file it works well, but in the second file there are two commas inserted after each number, something like this:

56,, 
74,,
2,,

How can I insert just one comma?

input_path = Path(Path.home(), "Desktop", "m")
for root, dirs, files in os.walk(input_path):
    for file in files:
        file_path = Path(root, file)
        
        with open(file_path) as f:
            lines = f.read().splitlines()
        with open('C:/--/{}.txt'.format(file), "w") as f:
            for line in lines:
                f.write(line + ",\n")