I am wondering how I can take only positive values in a list in Python.

For example, if I have `A = [1, 2, 3]`, it should return `[1, 2, 3]`

If I have `A = [-1, 2, 3]`, it should return `[2, 3]`

if I have `A = [-1, -2]`, it should return `None`

Thank you very much!