Preferred Python idiom for handling non-existing dictionary keys and why?

[Matt Goodall]

Quentin Crain wrote:

>Hello again All!
>
>(First, I would like to mention I did try to google
>for the answer here!)
>
>Say I am populating a dictionary with a list and
>appending. I have written it thusly:
>
>  d={}
>  for (k,v) in somedata():
>      try:
>          d[k].append(v)
>      except KeyError:
>          d[k]=[v]
>
>I could have written:
>
>  d={}
>  for (k,v) in somedata():
>      if (k in d):
>          d[k].append(v)
>      else:
>          d[k]=[v]
>
>
>Which is perferred and why? Which is "faster"?
>
I assume from the above code that somedata returns a sequence of (key, 
value) pairs, where a key may appear more than once, and you are trying 
to collect all the values for each key?

Dictionary objects have a very useful method, setdefault, to help with 
this sort of problem. I would write the above as:

 >>> somedata = (('a',1),('b',2),('a',3),('c',4))
 >>> d={}
 >>> for k,v in somedata:
...     d.setdefault(k,[]).append(v)
...
 >>> d
{'a': [1, 3], 'c': [4], 'b': [2]}


Cheers, Matt

-- 
Matt Goodall, Pollenation Internet Ltd
w: http://www.pollenation.net
e: matt at pollenation.net