I have a list comprehension that calls a function which potentially returns None.
>>> f = lambda x: x if x < 3 else None
>>> l = [f(x) for x in [1,2,3,4]]
[1, 2, None, None]
I'd like to have the list comprehension as above, without the 'None' entries.
What's a more efficient way to do the following, without creating additional overhead, and while retaining the efficiency of the list comprehension?
>>> filter(None, [f(x) for x in [1,2,3,4]])
[1, 2]
Add an if into your comprehension like:
l = [y for y in (f(x) for x in [1,2,3,4]) if y is not None]
By placing a Generator Expression inside the list comprehension you will only need to evaluate the function once. In addition the generator expression is a generator so takes no extra intermediate storage.
As of Python 3.8 you can use an Assignment Expression (:=) (AKA: Named Expressions or the Walrus Operator) to avoid multiple evaluations of f() like:
l = [y for x in [1,2,3,4] if (y := f(x)) is not None]