Here's some made up code: pruned = map( # Remove all elems whose foo is less than the number of elems lambda elems: list(filter( lambda elem: e

Here's some made up code: pruned = map( # Remove all elems whose foo is less than the number of elems lambda elems: list(filter( lambda elem: elem.foo < len(elems), elems )) ) Now let's say we want a running total of all the foos. We can insert an expression to do this: total_foos = [0] pruned = map( # Remove all elems whose foo is less than the number of elems lambda elems: list(filter( lambda elem: ( # Add elem.foo to our total_foos accumulator total_foos.append(total_foos.pop() + elem.foo), elem.foo < len(elems) )[-1], elems )) ) total_foos = total_foos.pop() This is rather awkward and "non-Pythonic"; ideally we would use 'total_foos += elem.foo', but that can't exist in a lambda. Hence: total_foos = 0 # Have to define this outside the map, since it's a statement def checkAndAccumulate(elems): """This checkAndAccumulate function is just a wrapper, for closing-over the elems variable (since it's not in-scope outside the map call). Returns the actual checking+accumulating function.""" def checker(elem): """The actual function we want to filter with""" total_foos += elem.foo return elem.foo < len(elems) return checker pruned = map( # Remove all elems whose foo is less than the number of elems lambda elems: list(filter( checkAndAccumulate(elems), elems )) )