List comprehension with if statement

list comprehension formula:

[<value_when_condition_true> if <condition> else <value_when_condition_false> for value in list_name]

thus you can do it like this:

[y for y in a if y not in b]

Only for demonstration purpose : [y if y not in b else False for y in a ]

You put the if at the end:

[y for y in a if y not in b]

List comprehensions are written in the same order as their nested full-specified counterparts, essentially the above statement translates to:

outputlist = []
for y in a:
    if y not in b:
        outputlist.append(y)

Your version tried to do this instead:

outputlist = []
if y not in b:
    for y in a:
        outputlist.append(y)

but a list comprehension must start with at least one outer loop.

You got the order wrong. The if should be after the for (unless it is in an if-else ternary operator)

[y for y in a if y not in b]

This would work however:

[y if y not in b else other_value for y in a]

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