[Python-ideas] Joining dicts again

[Mark Lawrence]

On 23/02/2014 13:31, haael at interia.pl wrote:
>
> @Steven D'Aprano
>
>> So we have *at least* four different ways to merge dictionaries a and b:
>>
>> # 1: a wins
>> c = b.copy()
>> c.update(a)
>>
>>
>> # 2: b wins
>> c = a.copy()
>> c.update(b)
>>
>>
>> # 3: choose a winner according to the `or` operator
>> c = a.copy()
>> for key, value in b.items():
>>      if key in c:
>>          c[key] = c[key] or value
>>      else:
>>          c[key] = value
>>
>>
>> # 4: keep both, in a list of 1 or 2 items
>> c = {key:[value] for key, value in a.items()}
>> for key, value in b.items():
>>      if key in c and value != c[key][0]:
>>          c[key].append(value)
>>      else:
>>          c[key] = [value]
>>
>>
>> The first three are special cases of a more general case, where
>> you have a "decision function" that takes two values (one from dict a
>> and the other from dict b) and decides which one to keep. Case 1 ("a
>> always wins") would use `lambda x,y: x`, case 2 ("b wins") would use
>> `lambda x,y: y` and case 3 would use operator.or_.
>>
>> The question is, why should any one of these be picked out as so
>> obviously more useful than the others as to deserve being a dict method
>> or operator support?
>>
>> Steven
>
>
>
> All solutions provided by you are not one-liners. Every requires at least 2 lines of code and is an imperative-style code block, instead of a simple expression.
>

Excellent as it makes the code easier to read.  Let's leave things like 
that.

> I would really like to have a simple dict joining _expression_ that can be inserted everywhere I just need.
>

I wouldn't.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask 
what you can do for our language.

Mark Lawrence

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