Avoid computing the same expression twice in list comprehension [duplicate]

Question

I am using a function in a list comprehension and an if function:

new_list = [f(x) for x in old_list if f(x) !=0]

It annoys me that the expression f(x) is computed twice in each loop.

Is there a way to do it in a cleaner way? Something along the lines of storing the value or including the if statement at the beginning of the list comprehension.

Answer 1

you could use a generator expression (in order to avoid creating an unnecessary list) inside your list comprehension:

new_list = [fx for fx in (f(x) for x in old_list) if fx != 0]

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starting from python 3.8 you will be able to do this:

new_list = [fx for x in old_list if (fx := f(x)) != 0]