scipy

import numpy as np
from scipy.optimize import curve_fit

import matplotlib.pyplot as plt

def func(x, a, b, c):
    return a * np.exp(-b*x) + c

x = np.linspace(0, 4, 50)
y = func(x, 2.5, 1.3, 0.5)
yn = y + 0.2 * np.random.normal(size=len(x))

popt, pcov = curve_fit(func, x, yn)

#print popt

plt.plot(x, yn, 'o', x, func(x, *popt))
plt.show()