API: Make ExtensionDtype.construct_array_type a method
This allows a single dtype to support multiple array classes.
For arrow-backed strings, we'll likely want a separate array class
for ease of implementation, clarity. But we'll have a parametrized
dtype.

```python
class StringDtype:
    def __init__(self, storage="python"):
        self.storage = storage

    def construct_array_type(self):  # regular method
        if self.storage == "python":
            return StringArray
        else:
            return ArrowStringArray
```

Closes #36126
I just had a thought... As long as *we're* careful to only call the method on instances and not types, it may be OK for StringDtype.construct_array_type to be a regular method? The only place we don't do that now is in the test_decimal tests, everywhere in the library we're already working on instances, not types.

It's not the most sound, but maybe it's OK to avoid any risk of API-breaking changes in the interface? Yes, that sounds correct to me. 

I would still document it that it is allowed? (it might also be useful for external dtypes) Or maybe add a based extension test to encourage downstream users to change it? Yeah, I think we'll want to encourage people to update. Perhaps via a (non-visisble) deprecationwarning at first, then an extension test asserting that it's not a classmethod, then we make the change in 2.x. > Yeah, I think we'll want to encourage people to update. Perhaps via a (non-visisble) deprecationwarning at first, then an extension test asserting that it's not a classmethod, then we make the change in 2.x.

I am ok with just changing this, its an experimental api after all, not worth waiting for 2.x Decimal tests failing @TomAugspurger just pushed a small commit that fixes the broken test Thanks. Need to think a bit more about the best way forward here. If pandas
doesn't actually use classmethods on instances, we may be able to get away
with just defining it as a regular method where we need it and not go
through a deprecation process.

On Thu, Sep 17, 2020 at 5:14 PM jbrockmendel <notifications@github.com>
wrote:

> @TomAugspurger <https://github.com/TomAugspurger> just pushed a small
> commit that fixes the broken test
>
> —
> You are receiving this because you were mentioned.
> Reply to this email directly, view it on GitHub
> <https://github.com/pandas-dev/pandas/pull/36136#issuecomment-694527627>,
> or unsubscribe
> <https://github.com/notifications/unsubscribe-auth/AAKAOIS5WLOV6A6PHVVWOPLSGKC4NANCNFSM4Q2YPRUA>
> .
>
 This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Going to hold on this till it's definitely needed. Personally, I think it is still useful to make this change. As long as we only call the method on instances, it shouldn't actually be a breaking change. So although it might not be explicitly needed for the StringArray (since it can just make it a plain method only there), it might still be useful in general to change this to signal that this is OK. 