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How Do I Efficiently Filter Computed Values Within A Python List Comprehension?

- 1 answer

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?

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Answer

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.

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source: stackoverflow.com
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