I have the following code:

mystring = ["reddit", "google"]
mylist = ["a", "b", "c", "d"]
for mystr in mystring:if any(x not in mystr for x in mylist):print(mystr)

I'm expecting that this should return only "google". But for some reason it returns both "reddit" and "google".

Your use of any and not in contradicts itself. You want to check either for all like this:

if all(x not in mystr for x in mylist):print mystr

Or just check for not any (which is more readable in my opinion):

if not any(x in mystr for x in mylist):print mystr

Both of these versions can be a one-liner by the way (instead of your loop), if you use list comprehension (but that is just a matter of taste, or if you prefer to print out one line of results instead of one line for every result):

mystring = ["reddit", "google"]
mylist = ["a", "b", "c", "d"]
print [s for s in mystring if not any(x in s for x in mylist)]