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

Question

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]
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Answer 1

This is exactly what generators are suited to handle:

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

1. This makes it totally clear what is happening during each stage of the pipeline.
2. Explicit over implicit
3. Uses generators everywhere until the final step, so no large intermediate lists

cf: 'Generator Tricks for System Programmers' by David Beazley