I have a method that takes a list and returns an object:
# input a list, returns an object
def map_to_obj(lst):
a_list = f(lst)
return a_list[0] if a_list else None
I want to get a list that contains all the mapped elements that aren't None.
Like this:
v_list = [v1, v2, v3, v4]
[map_to_obj(v) for v in v_list if map_to_obj(v)]
But it doesn't seem good to call the map_to_obj method twice in the list comprehension.
Is there a way to have local variables in list comprehensions so that it can have better performance?
Or does the compiler optimize it automatically?
Here is what I want:
(sml like)
[let mapped = map_to_obj(v) in for v in v_list if mapped end]
Starting in Python 3.8, and the introduction of assignment expressions (PEP 572) (:= operator), it's possible to use a local variable within a list comprehension in order to avoid calling the same function twice.
In our case, we can name the evaluation of map_to_obj(v) as a variable o while using the result of the expression to filter the list; and thus use o as the mapped value:
[o for v in [v1, v2, v3, v4] if (o := map_to_obj(v))]