The Python list comprehension syntax makes it easy to filter values within a comprehension. For example:
result = [x**2 for x in mylist if type(x) is int]
Will return a list of the squares of integers in mylist. However, what if the test involves some (costly) computation and you want to filter on the result? One option is:
result = [expensive(x) for x in mylist if expensive(x)]
This will result in a list of non-"false" expensive(x) values, however expensive() is called twice for each x. Is there a comprehension syntax that allows you to do this test while only calling expensive once per x?
If the calculations are already nicely bundled into functions, how about using filter and map?
result = filter (None, map (expensive, mylist))
You can use itertools.imap if the list is very large.