Question

I have to append elements to a list only if the current iterated element is not already in the list.

>>> l = [1, 2]
>>> for x in (2, 3, 4):
...     if x not in l:
...             l.append(x)
... 
>>> l
[1, 2, 3, 4]

vs

>>> l = [1, 2]
>>> [l.append(i) for i in (2, 3, 4) if i not in l]
[None, None]
>>> l
[1, 2, 3, 4]

The list comprehension gives the result is what I want, just the returned list is useless. Is this a good use case for list comprehensions?

The iteration is a good solution, but I’m wondering if there is a more idiomatic way to do this?

Answer 1

You could do:

l.extend((i for i in (2,3,4) if i not in l))

This solution still works if the added list is non-unique.