Add ExtesnsionDtype.empty classmethod

[TomAugspurger]

This would let us provide a default construct_from_string method.

It seems we could have a default implementation for ExtensionDtype.construct_from_string ? (I now just copy pasted from the decimal example, and I think json example also has the same basic one)

The default would have to rely on ExtensionDtype() being constructable with no arguments, or an ExtensionDtype.empty method. For many that'll be

@classmethod
def empty(cls):
    return cls()

which is easier than implementing construct_from_string. Maybe call it from_empty.

[jorisvandenbossche]

The default would have to rely on ExtensionDtype() being constructable with no arguments

Why not just rely on that assumption directly for a default implementation of construct_from_string ?
The we don't need a new method?

[TomAugspurger]

So if a class doesn't support ExtensionDtype() called with no arguments, we'll get an exception either way. My hope was that implementing .empty / .from_empty is easier than implementing .construct_from_string.

[jreback]

EA should have this method
maybe make dtype required?

[jorisvandenbossche]

What has this to do with ExtensionArray itself ?

[jreback]

sorry thought the top was on empty on EA