How to calculate the angle between a line and the horizontal axis?

In a programming language (Python, C#, etc) I need to determine how to calculate the angle between a line and the horizontal axis?

I think an image describes best what I want:

Given (P1_x,P1_y) and (P2_x,P2_y) what is the best way to calculate this angle? The origin is in the topleft and only the positive quadrant is used.