Python efficient way to check if very large string contains a substring

Question

Python is not my best language, and so I\'m not all that good at finding the most efficient solutions to some of my problems. I have a very large string (coming from a 30 MB

Answer 1

If you just want to check if that substring exists,

for line in open("file"):
    if substring in line:
         print "exists"
         sys.exit() # or break to do some other stuff