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
    ))
  )