On Mon, Dec 28, 2020 at 08:44:07AM -0000, Anton Abrosimov wrote:
> Steven D'Aprano wrote:
> > Why do you want something that isn't a mapping to be usable with mapping
> > unpacking?
>
> I think mapping is not `abc.Mapping` class only.
You don't have to inherit from Mapping for this to work. Double-star
unpacking already supports duck-typing:
>>> class MyMapping:
... def keys(self):
... return iter('abc')
... def __getitem__(self, key):
... if key in ('a', 'b', 'c'):
... return key.upper()
... raise KeyError
...
>>> def demo(**kwargs):
... print(kwargs)
...
>>> import collections.abc
>>> issubclass(MyMapping, collections.abc.Mapping)
False
>>>
>>> demo(**MyMapping())
{'a': 'A', 'b': 'B', 'c': 'C'}
So we already support duck-typing here.
We can't use the fallback iteration interface:
>>> class MyOtherMapping:
... def __iter__(self):
... return zip('xyz', 'XYZ')
...
>>> demo(**MyOtherMapping())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: __main__.demo() argument after ** must be a mapping, not
MyOtherMapping
so that's a possible enhancement: dict unpacking should(?) fall back on
iteration of (key, value) pairs just as dict.update does. I would
cautiously support adding that as an enhancement.
> What about:
> `Iterator[Tuple[str, int]]`
I said I would *cautiously* support that, because there are some subtle
issues to do with exceptions, but we can worry about that later.
> ```
> @dataclass
> class MyMap:
> x: int
> y: int
> ```
>
> Is this "mapping"?
No:
>>> issubclass(MyMap, collections.abc.Mapping)
False
It doesn't even duck-type as a mapping. It does not support len, keys or
subscripting:
>>> obj = MyMap(2, 3)
>>> len(obj)
TypeError: object of type 'MyMap' has no len()
>>> obj.keys()
AttributeError: 'MyMap' object has no attribute 'keys'
>>> obj['x']
TypeError: 'MyMap' object is not subscriptable
so it is certainly not a mapping.
--
Steve
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