Why is the IF condition being evaluated to True all the time

Question

Why the if statement being evaluated to True each time, even if I am deliberately giving biased inputs to my code . Here is my code:

s1 = 'efgh'
s2 = 'abcd'

for i in range(0, len(s1)):
    for j in range(1, len(s1)+1):
        if s1[i:j] in s2:
            print('YES')

It print YES, 6 times. Why is that?

Answer 1

Whenever you have i >= j, you will get an empty string for s1[i:j]. An empty string always returns True when checking in another string, hence your print statements.

Instead you should start j as i + 1:

s1 = 'efgh'
s2 = 'abcd'

for i in range(0,len(s1)):
    for j in range(i + 1,len(s1)+1):
        if s1[i:j] in s2:
            print('YES')

Which gives no output.

---

Empty strings are always considered to be a substring of any other string, so "" in "abc" will return True.

Docs source