for p in d.items():
...     key = p[0]
...     value = p[1]
...     if value not in ldoc:
...         ldoc[value] = [key]
...     else:
...         ldoc[value].append(key)
... 
>>> print ldoc
{1: ['orange', 'orange'], 2: ['pear', 'berry', 'pear', 'berry'], 3:
['cherry', 'apple', 'cherry', 'apple']}
>>> ldoc.clear()
>>> ldoc
{}
>>> def revdictionary(d):
...     rev = {}
...     return rev
... 
>>> print revdictionary(d)
{}
>>>