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

You could always memoize the expensive() function so that calling it the second time around is merely a lookup for the computed value of x.

Here's just one of many implementations of memoize as a decorator.