NTC Thermistor calculator

ntc.py

import math

def rp(a,b):
    "y = R1||R2"
    return a*b/float(a+b)

def rr(r,c):
    '''R||y = C'''
    return r*c/float(r-c)

class NTC:
    k = 273.15
    t = k + 25
    t_1 = 1.0/t
    def __init__(self, r, b):
        self.b = float(b)
        self.r = float(r)

    def t2r(self, t):
        t += self.k
        dt = 1.0/t - self.t_1
        x = self.b * dt
        r = self.r * math.exp(x)
        return r

    def r2t(self, r):
        a = r/self.r
        b = math.log(a)/self.b
        c = 1.0/(b + self.t_1)
        return c - self.k

    def __str__(self):
        msg = 'NTC: '
        msg += 'B = {}, R_25 = {:d}'.format(self.b, int(self.r))
        msg += ', ' + 'Tk = {}'.format(NTC.k)
        msg += ', ' + 'T_25 = {}'.format(NTC.t)
        msg += ', ' + 'T^-1 = {}'.format(NTC.t_1)
        return msg

if __name__ == '__main__':
    a = NTC(1e4,3950)
    print a
    x = a.t2r(18)
    print x
    print a.r2t(x)