Python atan or atan2, what should I use?

Atan takes single argument and Atan2 takes two arguments.The purpose of using two arguments instead of one is to gather information on the signs of the inputs in order to return the appropriate quadrant of the computed angle, which is not possible for the single-argument Atan

Atan2 result is always between -pi and pi.

Reference:
https://en.wikipedia.org/wiki/Atan2