Perlish dictionary behavior

[Robert Brewer]

Chris wrote:
> One nice thing about Perl that is helpful when tallying things up by
> type is that if you increment a hash key and the key does not exist,
> Perl puts a one there.  So you can have code that does something like
> this:
> 
> my %thingCounts;
> foreach my $thing(<file>)
> {
>   $thingCounts{$thing}++;
> }
>8
> Is there any clever way to be able to just say, like in Perl, 
> for thing in file:
>   thingCounts[thing] += 1
> 
> and have it do the above?  Perhaps with a custom dictionary class or
> something?  Just wondering what that might look like.

Here's one way:

>>> class AllThingsStartAtZero(dict):
... 	def __getitem__(self, key):
... 		return dict.get(self, key, 0)
... 	
>>> thingCounts = AllThingsStartAtZero()
>>> for thing in ('a', 'b', 'c', 'd', 'a'):
... 	thingCounts[thing] += 1
... 	
>>> thingCounts
{'a': 2, 'c': 1, 'b': 1, 'd': 1}

Notice that a actually does get 'incremented twice'.


Robert Brewer
MIS
Amor Ministries
fumanchu at amor.org