While I'm generally of the opinion that walrus shouldn't be used until py3.9's end of life, I'd steelman the second example: {x:y for x in data if (y :

While I'm generally of the opinion that walrus shouldn't be used until py3.9's end of life, I'd steelman the second example: {x:y for x in data if (y := f(x))} Benefit there is that you only iterate over data once (possibly exhasting it) and you only allocate the result list. In 3.9, that takes some wrangling and extra tuple creation/unpacking. pairs = ((x,func(x)) for x in data) result = {x:y for x,y in pairs if y}