这两个列表创建代码有什么区别（一个是带有 if 条件的普通 for 循环，另一个是线性代码）

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.