Selectively printing values from a python dictionary
Question: I have have a huge graph consisting of well over 100000 keys so efficiency is
a huge issue. I am going through every keys' value, and for every value, I
want it to be a key in another dictionary, with the values being the remaining
values... E.g...

    
    
    graph = {'foobar': ['1', '2', '3']}
    result = {'1' : ['2', '3'], '2' : ['1', '3'], '3' : ['1', '2']}  #in no particular order
    

Here is my code at the moment...

    
    
    for i in heroDict.values():
        for j in i:
            if graph.has_key(j):
                tempDict = copy.deepcopy(i)
                tempDict.remove(j)
                heroList = tempDict
                graph[j] += heroList
            else:
                tempDict = copy.deepcopy(i)
                tempDict.remove(j)
                heroList = tempDict
                graph[j] = heroList
    return graph
    

'heroDict' is a dictionary similar to the example except very very large.

The problem I am having is that my code is running very slowly because of the
deepcopy() I am performing. so for the foobar example for example, I get '1'
as a key. I copy ['1', '2', '3'] into a temporary dict so the changes to it
wont affect my final dictionary that i return. Then I remove the key from the
['1', '2', '3'] and assign the key '1' to it. So I'm left with {'1' : ['2',
'3']} which is what I want but its taking too long because its iterating
100000+ times.

My final question is, can I improve this in any way so it runs faster?

Any help is greatly appreciated.


Answer: Permutations is included in `itertools`.

A typical use in your example is :

    
    
    >>> from itertools import permutations
    >>> values = graph['foobar']
    >>> result = {x[0]:x[1:] for x in permutations(values)}
    >>> print result
    {'1': ('3', '2'), '2': ('3', '1'), '3': ('2', '1')}
    

Works with any number of values in foobar. Permutations is a generator, so you
may call one item at a time instead of generating the whole dict at once.

Not sure how fast that would be, though.

