How do I efficiently filter computed values within a Python list comprehension?

result = [x**2 for x in mylist if type(x) is int]

result = [expensive(x) for x in mylist if expensive(x)]

result = filter (None, map (expensive, mylist))

result = [y for y in (expensive(x) for x in mylist) if y]

result = [x for x in map(expensive,mylist) if x]

def gen_expensive(mylist):
    for item in mylist:
        result = expensive(item)
        if result:
            yield result

result = (expensive(x) for x in mylist)
result = (do_something(x) for x in result if some_condition(x))
...
result = [x for x in result if x]  # finally, a list

itertools.ifilter(bool, (expensive(x) for x in mylist))

result = []
for x in mylist:
    expense = expensive(x)
    if expense:
        result.append(expense)