Adding a “-” sign in each line of a .txt file using Python

Question

I have a text file with about 5 million lines and each line contains three numbers, like for example:

33.56 77.32 12.54

The numbers always have the same length.

Now I have to add a minus sign before the last number to get the following output:

33.56 77.32 -12.54

How can I do that in Python?

Answer 1

with open("filename.csv") as fh: 
    for line in fh: 
        line = line.strip()
        print(line[:12] + "-" + line[12:])