[Tutor] setdefault method

Kent Johnson kent37 at tds.net
Tue Mar 28 03:23:59 CEST 2006

Carroll, Barry wrote:
> Greetings:
> What is the purpose of the dictionary method setdefault(k[, x])?

setdefault() is perhaps badly named, but it is very useful. It doesn't 
do what you think it does! From the docs:
     setdefault() is like get(), except that if k is missing, x is both 
returned and inserted into the dictionary as the value of k. x defaults 
to None.

You could define your own like this (with an extra arg for the dict):
   def setdefault(d, k, x=None):
     if k in d:
       return d[k]
       d[k] = x
       return x

If k is in d, the existing value d[k] is returned. If k is not in d, the 
magic happens - d[k] is set to x, and x is returned to you.

I find this most often useful when I want to make a dict that maps a key 
to a list of values. For example I may have a list of key, value pairs 
and I want to accumulate the list of all values for each key. This comes 
up pretty often in my experience. Here is how to code it with setdefault():

d = {}
for k, v in some_list:
   d.setdefault(k, []).append(v)

Here setdefault() will return the list that k maps to, if any, or 
otherwise start a new list. In either case, you can append the new value 
to the returned list and get the effect you want.

> And, if d.setdefault does not actually assign a default value for d, is there a way to do this?

See this recipe:


More information about the Tutor mailing list