Nagy László Zsolt writes:
> The result that I need should be a real dict, not just a ChainMap. (It
> is because I have to mutate it.)
>
> d1 = {'a':1, 'b':2}
> d2 = {'c':3, 'd':4}
> d3 = {'e':5, 'f':6}
>
> #1. My first naive approach was:
>
>
> from collections import ChainMap
> d = {}
> for key,value in ChainMap(d1, d2, d3).items():
> d[key] = value
>
> #2. Much more effective version, but requires too many lines:
>
> d= {}
> d.update(d1)
> d.update(d2)
> d.update(d3)
>
> #3. Third version is more compact. It uses a side effect inside a list
> comp., so I don't like it either:
>
> d = {}
> [d.update(_) for _ in [d1, d2, d3]]
That really should be just a loop:
d = {}
for _ in (d1, d2, d3):
d.update(_)
> #4. Last version:
>
> d = {}
> d.update(ChainMap(d1, d2, d3))
>
> Visually, it is the cleanest and the easiest to understand. However, it
> uses ChainMap.__iter__ and that goes over all mappings in a loop written
> in pure Python.
>
> Is there a version that is as effective as #3, but as clean and nice
> as #4?
Maybe see above.
Or consider also this, which looks straightforward to me:
d = { k:v for d in (d1, d2, d3) for k,v in d.items() }
Is ChainMap really that bad? Otherwise the following would look somewhat
nice:
d = dict(ChainMap(d1, d2, d3).items())
Those come to mind.
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