dict invert - learning question

[Arnaud Delobelle]

dave <squareswallower at y1a2hoo3.com> writes:

> Hello,
>
> here is a piece of code I wrote to check the frequency of values and
> switch them around to keys in a new dictionary.  Just to measure how
> many times a certain key occurs:
>
> def invert(d):
> 	inv = {}
> 	for key in d:
> 		val = d[key]
> 		if val not in inv:
> 			inv.setdefault(val, [key])
You can simply write:
                        inv[val] = [key]
> 		else:
> 			inv[val].append(key)
> 	return inv
>
>
> Using the methods above (I'm just a beginner) could I have written it
> more concisely?  Any criticism/critique in the code would be greatly
> appreciated.

Apart from the unnecessary use of setdefault, it looks good to me.

* You could change if 'val not in inv:' to 'if val in inv:' (and swap
  the if and else clauses of course) in order to have a positive
  condition rather than a negative one
 
* If you want to use setdefault, you can replace the if .. else
  construct by:

               inv.setdefault(val, []).append(key)

* You can also iterate over keys and values using the items() or
  iteritems() method of dictionaries:

def invert(d):
    inv = {}
    for key, val in d.iteritems():
        inv.setdefault(val, []).append(key)
    return inv

-- 
Arnaud