R = 4.68; Ke = 0.057; Kc = 0.057; J = 1.95e-5
U = 20; tau = R*J/(Ke*Kc)

def iteration(yi,pas,tau,yf):
    return yi+pas/tau *(yf-yi)

def euleur(tau,y0,yf,tf,nb):
    t=0
    y=y0
    pas = tf/nb
    tl,yl=[t],[y]
    while t < tf:
        y = iteration(y,pas,tau,yf)
        yl.append(y)
        t+= pas
        tl.append(t)
    return tl,yl

import matplotlib.pyplot as plt
X,Y = euleur(tau,0,U/Ke,0.25,1000)
plt.plot(X,Y)
plt.show()