Posts tagged ‘setdefault’

Python’s setdefault Considered Harmful

2012-01-28 19:26

Python dictionaries have an inconspicuous method named setdefault. Asking for its description, we’ll be presented with rather terse interpretation:

  1. >>> help(dict.setdefault)
  3. setdefault(...)
  4.     D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D

While it might not be immediately obvious what we could use this method for, we can actually find quite a lot of applications if we only pay a little attention. The main advantage of setdefault seems to lie in elimination of ifs; a feat we can feel quite smug about. As an example, consider a function which groups list of key-value pairs (with possible duplicate keys) into a dictionary of lists:

  1. def group_assoc_list_(a_list): # with if
  2.     res = {}
  3.     for k, v in a_list:
  4.         if k in res:
  5.             res[k].append(v)
  6.         else:
  7.             res[k] = [v]
  8.     return res
  10. def group_assoc_list(a_list): # with setdefault
  11.     res = {}
  12.     for k, v in a_list:
  13.         res.setdefault(k, []).append(v)
  14.     return res

This is something which we could do when parsing query string of an URL, if there weren’t a readily available API for that:

  1. def parse_query_string(qs):
  2.     return [arg.split('=') for arg in qs.split('&')]
  4. >>> group_assoc_list(parse_query_string('a=1&b=2&c=3&b=4&a=5'))
  5. {'a': ['1', '5'], 'b': ['2', '4'], 'c': ['3']}

So, with setdefault we can get the same job done more succinctly. It would seem that it is a nifty little function, and something we should keep in mind as part of our toolbox. Let’s remember it and move on, shall we?…

Actually, no. setdefault is really not a good piece of dict‘s interface, and the main reason we should remember about it is mostly for its caveats. Indeed, there are quite a few of them, enough to shrink the space of possible applications to rather tiny size. As a result, we should be cautious whenever we see (or write) setdefault in our own code.

Here’s why.


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