How to convert a negative number to positive?

>>> n = -42
>>> -n       # if you know n is negative
42
>>> abs(n)   # for any n
42

Don't forget to check the docs.

If "keep a positive one" means you want a positive number to stay positive, but also convert a negative number to positive, use abs():

>>> abs(-1)
1
>>> abs(1)
1

simply multiplying by -1 works in both ways ...

>>> -10 * -1
10
>>> 10 * -1
-10