get all the letters in Python

How do I get all the letters in Python?

if we want to know how many letters are used in the string then we can take help of set ( ) function which will remove repetition of letters. let’s see example of it-

x="this is our first line" 
s=set(()) 
for i in range(len(x)): 
if x[i]==" ": 
pass 
else: 
s.add('%s'%x[i])

print(s)