Oh thanks, I had no idea that existed.
I wanted to experiment with Mapping and unions but it's not working. I
couldn't find any existing issues about this. Should I be opening an issue?
If not then there's no need to explain what I'm doing wrong.
```
$ cat test.py
print({} | {})
$ ./python test.py
{}
$ mypy --python-version 3.9 test.py
test.py:1: error: Unsupported left operand type for | ("Dict[<nothing>,
<nothing>]")
Found 1 error in 1 file (checked 1 source file)
$ mypy --version
mypy 0.782
```
On Wed, Aug 26, 2020 at 11:18 PM Guido van Rossum <gu...@python.org> wrote:
> Use typing.AbstractSet for the ABC (there was an unfortunate naming
> conflict -- we have dict and Mapping, list and Sequence, but set and Set...)
>
> On Wed, Aug 26, 2020 at 2:16 PM Alex Hall <alex.moj...@gmail.com> wrote:
>
>> On Wed, Aug 26, 2020 at 10:10 PM Christopher Barker <python...@gmail.com>
>> wrote:
>>
>>> Honestly, I haven't delved into type checking much, so maybe I'm making
>>> this up, but what if you write a function that is type hinted to take a
>>> Mapping, and uses | or |=, and all the test code uses a built in dict, and
>>> all seems to be well.
>>>
>>
>> This made me curious so I did an experiment, but with Set because I
>> couldn't be bothered to worry about Python versions. This code:
>>
>> ```
>>
>> import typing
>> import collections
>> import builtins
>>
>> def foo(t: typing.Set, c: collections.Set, b: builtins.set):
>> print(t + t)
>> print(t | t)
>> print(t.union(t))
>>
>> print(c + c)
>> print(c | c)
>> print(c.union(c))
>>
>> print(b + b)
>> print(b | b)
>> print(b.union(b))
>>
>> ```
>>
>> complains about `+` (as it should) and `c.union` in both PyCharm and
>> mypy, and nothing else. So typing.Set is based on the builtin and not the
>> ABC. If you want to support any subclass of collections.Set, your type hint
>> needs to be collections.Set. Using typing.Set will fail to show warnings
>> when you use a builtin only method, and will show warnings when you try to
>> assign an ABC set (e.g. I get a warning with `foo(t=<instance of
>> collections.abc.Set>)`. The problem then is that `collections.abc.Set[int]`
>> doesn't work, i.e. you need to use typing.Set if you want generics.
>>
>
>
> --
> --Guido van Rossum (python.org/~guido)
> *Pronouns: he/him **(why is my pronoun here?)*
> <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
>
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