Consider the following list comprehension
[ (x,f(x)) for x in iterable if f(x) ]
This filters the iterable based a condition f and returns the pairs of x,f(x). The problem with this approach is f(x) is calculated twice.
It would be great if we could write like
[ (x,fx) for x in iterable if fx where fx = f(x) ]
or
[ (x,fx) for x in iterable if fx with f(x) as fx ]
But in python we have to write using nested comprehensions to avoid duplicate call to f(x) and it makes the comprehension look less clear
[ (x,fx) for x,fx in ( (y,f(y) for y in iterable ) if fx ]
Is there any other way to make it more pythonic and readable?
Update
Coming soon in python 3.8! PEP
# Share a subexpression between a comprehension filter clause and its output
filtered_data = [y for x in data if (y := f(x)) is not None]