<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def compare_dicts(old=None, new=None):
    '''
    Compare before and after results from various salt functions, returning a
    dict describing the changes that were made.
    '''
    ret = {}
    for key in set((new or {})).union((old or {})):
        if key not in old:
            # New key
            ret[key] = {'old': '',
                        'new': new[key]}
        elif key not in new:
            # Key removed
            ret[key] = {'new': '',
                        'old': old[key]}
        elif new[key] != old[key]:
            # Key modified
            ret[key] = {'old': old[key],
                        'new': new[key]}
    return ret