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Re: Keeping track of things with dictionaries

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"Chris Angelico" <rosuav@gmail.com> wrote in message 
news:CAPTjJmqFBt2XX+BDfNHz0gaGOrDkhtpBzrR29DUWN36girzcSw@mail.gmail.com...
> On Tue, Apr 8, 2014 at 2:02 PM, Josh English <Joshua.R.English@gmail.com> 
> wrote:
>>
>> Would dict.setdefault() solve this problem? Is there any advantage to 
>> defaultdict over setdefault()
>
> That depends on whether calling Brand() unnecessarily is a problem.
> Using setdefault() is handy when you're working with a simple list or
> something, but if calling Brand() is costly, or (worse) if it has side
> effects that you don't want, then you need to use a defaultdict.
>

It appears that when you use 'setdefault', the default is always evaluated, 
even if the key exists.

>>> def get_value(val):
...   print('getting value', val)
...   return val*2
...
>>> my_dict = {}
>>> my_dict.setdefault('a', get_value('xyz'))
getting value xyz
'xyzxyz'
>>> my_dict.setdefault('a', get_value('abc'))
getting value abc
'xyzxyz'
>>> my_dict
{'a': 'xyzxyz'}
>>>

It seems odd. Is there a situation where this behaviour is useful?

Frank Millman

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Thread

Keeping track of things with dictionaries Giuliano Bertoletti <gbe32241@libero.it> - 2014-04-06 09:44 +0200
  Re: Keeping track of things with dictionaries Peter Otten <__peter__@web.de> - 2014-04-06 10:23 +0200
  Re: Keeping track of things with dictionaries Josh English <Joshua.R.English@gmail.com> - 2014-04-07 21:02 -0700
    Re: Keeping track of things with dictionaries Chris Angelico <rosuav@gmail.com> - 2014-04-08 14:08 +1000
      Re: Keeping track of things with dictionaries Josh English <Joshua.R.English@gmail.com> - 2014-04-07 23:22 -0700
    Re: Keeping track of things with dictionaries "Frank Millman" <frank@chagford.com> - 2014-04-08 09:14 +0200
      Re: Keeping track of things with dictionaries Steven D'Aprano <steve@pearwood.info> - 2014-04-08 07:47 +0000
    Re: Keeping track of things with dictionaries Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-08 01:53 -0600
    Re: Keeping track of things with dictionaries Chris Angelico <rosuav@gmail.com> - 2014-04-08 18:00 +1000
    Re: Keeping track of things with dictionaries Peter Otten <__peter__@web.de> - 2014-04-08 10:21 +0200
    Re: Keeping track of things with dictionaries "Frank Millman" <frank@chagford.com> - 2014-04-08 10:26 +0200
    Re: Keeping track of things with dictionaries "Frank Millman" <frank@chagford.com> - 2014-04-08 10:31 +0200
      Re: Keeping track of things with dictionaries alex23 <wuwei23@gmail.com> - 2014-04-09 12:34 +1000
    Re: Keeping track of things with dictionaries Chris Angelico <rosuav@gmail.com> - 2014-04-08 18:35 +1000
      Re: Keeping track of things with dictionaries Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-09 12:43 +1200
        Re: Keeping track of things with dictionaries Chris Angelico <rosuav@gmail.com> - 2014-04-09 12:33 +1000
          Re: Keeping track of things with dictionaries alex23 <wuwei23@gmail.com> - 2014-04-09 12:45 +1000
            Re: Keeping track of things with dictionaries Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-08 21:19 -0600
            Re: Keeping track of things with dictionaries Gene Heskett <gheskett@wdtv.com> - 2014-04-08 23:31 -0400
            Re: Keeping track of things with dictionaries Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-08 21:37 -0600
    Re: Keeping track of things with dictionaries "Frank Millman" <frank@chagford.com> - 2014-04-08 11:28 +0200
    Re: Keeping track of things with dictionaries Chris Angelico <rosuav@gmail.com> - 2014-04-08 19:34 +1000
    Re: Keeping track of things with dictionaries "Frank Millman" <frank@chagford.com> - 2014-04-08 11:41 +0200
  Re: Keeping track of things with dictionaries Gene Heskett <gheskett@wdtv.com> - 2014-04-09 05:51 -0400

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