[Tutor] Sort dictionaries

[Kent Johnson]

David Holland wrote:
 > Is there a built command to sort a dictionary keys or
 > values by their length or do I have to write a
 > function to do it myself ?

In Python 2.4 you can use sort with the builtin len as the key:
 >>> d = dict(aaa='aaa', bb='bb', c='c')
 >>> d
{'c': 'c', 'aaa': 'aaa', 'bb': 'bb'}
 >>> k=d.keys()
 >>> k
['c', 'aaa', 'bb']
 >>> k.sort(key=len)
 >>> k
['c', 'bb', 'aaa']

In Python 2.3 the usual way to do this is with the 
'decorate-sort-undecorate' idiom:
Make a helper list containing tuples. The first element of the tuple is 
what you want to sort on, the second element is the actual data:
 >>> l = [ (len(key), key) for key in d.keys() ]
 >>> l
[(1, 'c'), (3, 'aaa'), (2, 'bb')]

Sort the helper list:
 >>> l.sort()
 >>> l
[(1, 'c'), (2, 'bb'), (3, 'aaa')]

Extract the original list:
 >>> [ key for (keylen, key) in l]
['c', 'bb', 'aaa']

You could also write a comparison function and pass it to the sort, but 
that is not recommended in general, it is much slower than DSU.

BTW this is really a question about lists, not dictionaries - d.keys() 
is a list, as are d.values() and d.items().

Kent