Monday, October 6, 2014

Python: add an element to an array only if a filter function returns True

This post was going to be about a fairly obscure feature of Python I found, but is now a minor cautionary tale about trying to be too clever :)

I was looking for an elegant solution to the problem of appending elements to a list only if a filter function returns true. The long (and in retrospect much more readable) way to write this is something like:
filter_result = self.FilterFunction(response)
if filter_result:
There is in fact a one-liner that can do this for you, but since its fairly obscure it makes the code much harder to understand.
processed_responses += filter(None, [self.FilterFunction(response)])
This works because when the first argument to filter is None, the effect is to remove all items from the sequence that evaluate to False. In this case that means if self.FilterFunction is False you get an empty array, and appending the empty array has no effect on processed_responses. If it's True, you append a single element.

Obvious huh?

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