ex.py

def check_chaine(chaine) :
 tab = list(chaine)
 i = 0
 while i < len(chaine):
 if tab[i] != 'a' and tab[i] != 't' and tab[i] != 'g' and tab[i] != 'c' :
 error = 1
 print ('error')
 break
 else :
 i+=1

error = 0
adn = input("Enter chain : ")
check_chaine(adn)
if error == 1:
 exit

minichain = input("Enter sequence : ")

rs = 0
save = 0
while rs < len(adn) or rs == 0:
 print ("ok");
 print (rs)
 save = rs
 
 rs = adn.find(minichain, rs)
 if rs <= save :
 exit