What is the difference between these two list creation codes (one normal for loop with if condition and the other one liner code)

Question

The first code which creates an array with no duplicate members from a string, using a normal for loop and an if condition.

u = []
for l in string:
    if l not in u:
        u.append(l)

And the second code which does the same thing but in one line.

u = []
u = [l for l in string if l not in u]

The condition in the one line code is not working and at the end, u always contains all the characters in string.

Answer 1

With the list comprehension approach, the object u in the condition is fixed as the empty list [] so the condition will always be satisfied:

u = [l for l in string if l not in u] # here u in the condition is always []

With the first approach, the condition if l not in u always see the updated u so it doesn't add the element if it is already present.