Type ExtensionArray.view, ExtensionArray.astype, ExtensionArray.fillna and ExtensionArray.searchsorted

[MarcoGorelli]

• Closes #xxxx (Replace xxxx with the Github issue number)
• Tests added: Please use assert_type() to assert the type of any return value

[cmp0xff]

• It adds types to .astype, .fillna, .view and .searchsorted with tests (addresses #1317)
• It also adds Any to .tolist and adds default values ._reduce
• Just minor comment, otherwise good to go

[cmp0xff]

I would try

Suggested change

	def astype(self, dtype: ExtensionDtype, copy: bool = True) -> ExtensionArray: ...
	def astype(self, dtype: ExtensionDtype, copy: bool = True) -> Self: ...

because I guess you can't convert an IntervalArray to an IntegerArray by .astype. But I understand that this is unspecified in the docs, so keeping -> ExtensionArray is also fine.

[MarcoGorelli]

I think it possible though?

In [34]: arr = pd.Series([1.]).array

In [35]: arr
Out[35]:
<NumpyExtensionArray>
[1.0]
Length: 1, dtype: float64

In [36]: arr.astype('Int64')
Out[36]:
<IntegerArray>
[1]
Length: 1, dtype: Int64

[cmp0xff]

I see, my assumption was not correct.

[cmp0xff]

Looks good to me. I hope one day we can get rid of -> ExtensionArray, but we can leave it for another project. Thank you @MarcoGorelli !

[cmp0xff]

I see, my assumption was not correct.