Trapezoidal approximation error plotting in python - python-3.x

Question

Im trying to code a function that plots the error of the composite trapezoidal rule against the step size.
Obviously this doesn't look to good since i'm just starting to learn these things.
Anyhow i managed to get the plot and everything, but i'm supposed to get a plot with slope 2, so i am in need of help to figure out where i did go wrong.
from scipy import *
from pylab import *
from matplotlib import *
def f(x): #define function to integrate
return exp(x)
a=int(input("Integrate from? ")) #input for a
b=int(input("to? ")) #inpput for b
n=1
def ctrapezoidal(f,a,b,n): #define the function ctrapezoidal
h=(b-a)/n #assign h
s=0 #clear sum1-value
for i in range(n): #create the sum of the function
s+=f(a+((i)/n)*(b-a)) #iterate th sum
I=(h/2)*(f(a)+f(b))+h*s #the function + the sum
return (I, h) #returns the approximation of the integral
val=[] #start list
stepsize=[]
error=[]
while len(val)<=2 or abs(val[-1]-val[-2])>1e-2:
I, h=ctrapezoidal(f,a,b,n)
val.append(I)
stepsize.append(h)
n+=1
for i in range(len(val)):
error.append(abs(val[i]-(e**b-e**a)))
error=np.array(error)
stepsize=np.array(stepsize)
plt.loglog(stepsize, error, basex=10, basey=10)
plt.grid(True,which="both",ls="steps")
plt.ylabel('error')
plt.xlabel('h')