Question

I want to construct a value in a list comprehension, but also filter on that value. For example:

[expensive_function(x) for x in generator where expensive_function(x) < 5]

I want to avoid calling expensive_function twice per iteration.

The generator may return an infinite series, and list comprehensions aren’t lazily evaluated. So this wouldn’t work:

[y in [expensive_function(x) for x in generator where expensive_function(x)] where y < 5]

I could write this another way, but it feels right for a list comprehension and I’m sure this is a common usage pattern (possible or not!).

Answer 1

If generator may be infinite, you do not want to use a list comprehension. And not everything has to be a one-liner.

def filtered_gen(gen):
    for item in gen:
        result = expensive_function(item)
        if result < 5:
            yield result