A simple PID(proportional–integral–derivative) controller implementation.

pid.py

class PidController(object):
    def __init__(self, kp=1.0, ki=0.0, kd=0.0, setpoint=0.0, integrator_range=0.0):
        self.reset()
        self.integrator_range = integrator_range
        self.integrator_max = abs(integrator_range)
        self.integrator_min = -1 * self.integrator_max;
        self.kp = kp
        self.ki = ki
        self.kd = kd
        self.set_setpoint(setpoint)
        
    def set_setpoint(self, value):
        self.setpoint = value
        self.integrator = 0.0
        self.derivator = 0.0
    
    def reset(self):
        self.setpoint = 0.0
        self.integrator = 0.0
        self.derivator = 0.0
    
    def update(self, data):
        # Calculate error.
        err = self.setpoint - float(data)
        p = self.kp * err
        # Calculate propotional part.
        d = self.kd * (err - self.derivator)
        self.derivator = err
        # Calculate integral part.
        if self.integrator_range == 0.0:
            self.integrator = err + self.integrator
        else:
            self.integrator = min(self.integrator_max, max(self.integrator_min, err + self.integrator))
        i = self.integrator * self.ki
        # Summerize P.I.D.
        return p + i + d