these are the 2 python3 code,

{w for w in words if w[::-1] in words and len(w)==4}

{w for w in words if w==w[::-1]and len(w)==4}

In my point, w is the word in words, so w should equal with words，but the output is differently. Could someone help me, why the output is different?

>Solution :

if words is {'abba', 'abcd', 'dcba'} then the result of the first code will be {'abba', 'abcd', 'dcba'} while the second will return {'abba'}.

w==w[::-1] checks if the word is equal to its reverse while w[::-1] in words checks if the reverse of w is in words.