[Python-Dev] Another case for frozendict

[Mark Roberts]

I find it handy to use named tuple as my database mapping type.  It allows you to perform this behavior seamlessly.

-Mark

> On Jul 13, 2014, at 7:04, "Jason R. Coombs" <jaraco at jaraco.com> wrote:
> 
> I repeatedly run into situations where a frozendict would be useful, and every time I do, I go searching and find the (unfortunately rejected) PEP-416. I’d just like to share another case where having a frozendict in the stdlib would be useful to me.
>  
> I was interacting with a database and had a list of results from 206 queries:
>  
> >>> res = [db.cases.remove({'_id': doc['_id']}) for doc in fives]
> >>> len(res)
> 206
>  
> I can see that the results are the same for the first two queries.
>  
> >>> res[0]
> {'n': 1, 'err': None, 'ok': 1.0}
> >>> res[1]
> {'n': 1, 'err': None, 'ok': 1.0}
>  
> So I’d like to test to see if that’s the case, so I try to construct a ‘set’ on the results, which in theory would give me a list of unique results:
>  
> >>> set(res)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: unhashable type: 'dict'
>  
> I can’t do that because dict is unhashable. That’s reasonable, and if I had a frozen dict, I could easily work around this limitation and accomplish what I need.
>  
> >>> set(map(frozendict, res))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> NameError: name 'frozendict' is not defined
>  
> PEP-416 mentions a MappingProxyType, but that’s no help.
>  
> >>> res_ex = list(map(types.MappingProxyType, res))
> >>> set(res_ex)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: unhashable type: 'mappingproxy'
>  
> I can achieve what I need by constructing a set on the ‘items’ of the dict.
>  
> >>> set(tuple(doc.items()) for doc in res)
> {(('n', 1), ('err', None), ('ok', 1.0))}
>  
> But that syntax would be nicer if the result had the same representation as the input (mapping instead of tuple of pairs). A frozendict would have readily enabled the desirable behavior.
>  
> Although hashability is mentioned in the PEP under constraints, there are many use-cases that fall out of the ability to hash a dict, such as the one described above, which are not mentioned at all in use-cases for the PEP.
>  
> If there’s ever any interest in reviving that PEP, I’m in favor of its implementation.
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