append a string to a modified line

[Axel_Erfurt]

change /path/to/playlist.m3u and /path/to/new_playlist.m3u to your needs.

in_text = open("/path/to/playlist.m3u", "r").read()

out_text = ""

for line in in_text.splitlines():
    if line.startswith("#EXTINF"):
        out_text +=  line
        name = line.split(",", 1)[1]
        out_text += f'\n{name}.mp3\n'
    else:
        out_text += f"{line}\n"
        
print(out_text)

with open("/path/to/new_playlist.m3u", "w") as f:
    f.write(out_text)