missing? dictionary methods

Antoon Pardon apardon at forel.vub.ac.be
Mon Mar 21 13:34:25 CET 2005


Op 2005-03-21, Robert Kern schreef <rkern at ucsd.edu>:
> Antoon Pardon wrote:
>> Well at least I find them missing.
>> 
>> For the moment I frequently come across the following cases.
>> 
>> 1) Two files, each with key-value pairs for the same dictionary.
>> However it is an error if the second file contains a key that
>> was not in the first file.
>> 
>> In treating the second file I miss a 'set' method.
>> dct.set(key, value) would be equivallent to dct[key] = value,
>> except that it would raise a KeyError if the key wasn't
>> already in the dictionary.
>> 
>> 
>> 2) One file with key-value pairs. However it is an error
>> if a key is duplicated in the file.
>> 
>> In treating such files I miss a 'make' method.
>> dct.make(key, value) would be equivallent to dct[key] = value.
>> except that it would raise a KeyError if the key was
>> already in the dictionary.
>> 
>> 
>> What do other people think about this?
>
> def safeset(dct, key, value):
>      if key not in dct:
>          raise KeyError(key)
>      else:
>          dct[key] = value
>
> def make(dct, key, value):
>      if key in dct:
>          raise KeyError('%r already in dict' % key)
>      else:
>          dct[key] = value
>
> I don't see a good reason to make these built in to dict type.
>

I would say the same reason that we have get. There is no
reason to have a builtin get it is easily implemented
like this:

  def get(dct, key, default):

    try:
      return dct[key]
    except KeyError:
      return default


I would go even so far that there is more reason to have a built-in
safeset and make, than there is a reason to have a built-in get.

The reason is that a python implementation of safeset and make,
will mean two accesses in the dictionary, once for the test and
once for the assignment. This double access could be eliminated
with a built-in. The get on the other hand does only one dictionary
access, so having it implemeted in python is a lesser burden.

-- 
Antoon Pardon



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