Iterates over layout objects pointed in `self.slice` executing `function` on them.
        It passes `function` penultimate layout object and the position where to find last one

def pre_map(self, function):
        """
        Iterates over layout objects pointed in `self.slice` executing `function` on them.
        It passes `function` penultimate layout object and the position where to find last one
        """
        if isinstance(self.slice, slice):
            for i in range(*self.slice.indices(len(self.layout.fields))):
                function(self.layout, i)

        elif isinstance(self.slice, list):
            # A list of pointers  Ex: [[[0, 0], 'div'], [[0, 2, 3], 'field_name']]
            for pointer in self.slice:
                position = pointer[0]

                # If it's pointing first level
                if len(position) == 1:
                    function(self.layout, position[-1])
                else:
                    layout_object = self.layout.fields[position[0]]
                    for i in position[1:-1]:
                        layout_object = layout_object.fields[i]

                    try:
                        function(layout_object, position[-1])
                    except IndexError:
                        # We could avoid this exception, recalculating pointers.
                        # However this case is most of the time an undesired behavior
                        raise DynamicError("Trying to wrap a field within an already wrapped field, \
                            recheck your filter or layout")