Like I said in my comment, this is really vague. But I'll try to explain a little about the concept of a filter in python, specifically the filter() function.
The prototype of filter is: iterable <- filter(function, iterable).
iterable is something that can be iterated over. You can look up this term in the docs for a more exact explanation, but for your question, just know that a list is iterable.
function is a function that accepts a single element of the iterable you specify (in this case, an element of the list) and returns a boolean specifying whether the element should exist in the iterable that is returned. If the function returns True, the element will appear in the returned list, if False, it will not.
Here's a short example, showing how you can use the filter() function to filter out all even numbers (which I should point out, is the same as "filtering in" all odd numbers)
def is_odd(i): return i%2
l = [1,2,3,4,5] # This is a list
fl = filter(is_odd, l)
print fl # This will display [1,3,5]
You should convince yourself that is_odd works first. It will return 1 (=True) for odd numbers and 0 (=False) for even numbers.
In practice, you usually use a lambda function instead of defining a single-use top-level function, but you shouldn't worry about that, as this is just fine.
But anyway, you should be able to do something similar to accomplish your goal.
process_filter_description()? or is that what you are going to write?images.