Don't type check __getattr__?

Nobody has claimed this yet.

In #4592 I had the issue that mypy did not raise an error on a missing method:

from xarray.core.common import DataWithCoords

hasattr(xr.core.common.DataWithCoords, "reduce") # -> False

def test(x: "DataWithCoords"):
    x.reduce() # mypy does not error

This is because DataWithCoords implements __getattr__:


class A:
    pass

class B:
    def __getattr__(self, name):
        ...

def testA(x: "A"):
    x.reduce() # mypy errors

def testB(x: "B"):
    x.reduce() # mypy does not error

The solution seems to be to not typecheck __getattr__ (see https://github.com/python/mypy/issues/6251#issuecomment-457287161):

from typing import no_type_check

class C:
    @no_type_check
    def __getattr__(self, name):
        ...

def testC(x: "C"):
    x.reduce() # mypy errors

The only __getattr__ within xarray is here:

https://github.com/pydata/xarray/blob/17358922d480c038e66430735bf4c365a7677df8/xarray/core/common.py#L221

Using @no_type_check leads to 24 errors and not all of them can be trivially solved. E.g. DataWithCoords wants of use self.isel but does not implement the method. The solution is probably to add isel to DataWithCoords as an ABC or using NotImplemented.

Thoughts?

All errors

xarray/core/common.py:370: error: "DataWithCoords" has no attribute "isel"
xarray/core/common.py:374: error: "DataWithCoords" has no attribute "dims"
xarray/core/common.py:378: error: "DataWithCoords" has no attribute "indexes"
xarray/core/common.py:381: error: "DataWithCoords" has no attribute "sizes"
xarray/core/common.py:698: error: "DataWithCoords" has no attribute "_groupby_cls"
xarray/core/common.py:761: error: "DataWithCoords" has no attribute "_groupby_cls"
xarray/core/common.py:866: error: "DataWithCoords" has no attribute "_rolling_cls"; maybe "_rolling_exp_cls"?
xarray/core/common.py:977: error: "DataWithCoords" has no attribute "_coarsen_cls"
xarray/core/common.py:1108: error: "DataWithCoords" has no attribute "dims"
xarray/core/common.py:1109: error: "DataWithCoords" has no attribute "dims"
xarray/core/common.py:1133: error: "DataWithCoords" has no attribute "indexes"
xarray/core/common.py:1144: error: "DataWithCoords" has no attribute "_resample_cls"; maybe "resample"?
xarray/core/common.py:1261: error: "DataWithCoords" has no attribute "isel"
xarray/core/alignment.py:278: error: "DataAlignable" has no attribute "copy"
xarray/core/alignment.py:283: error: "DataAlignable" has no attribute "dims"
xarray/core/alignment.py:286: error: "DataAlignable" has no attribute "indexes"
xarray/core/alignment.py:288: error: "DataAlignable" has no attribute "sizes"
xarray/core/alignment.py:348: error: "DataAlignable" has no attribute "dims"
xarray/core/alignment.py:351: error: "DataAlignable" has no attribute "copy"
xarray/core/alignment.py:353: error: "DataAlignable" has no attribute "reindex"
xarray/core/alignment.py:356: error: "DataAlignable" has no attribute "encoding"
xarray/core/weighted.py:157: error: "DataArray" has no attribute "notnull"
xarray/core/dataset.py:3792: error: "Dataset" has no attribute "virtual_variables"
xarray/core/dataset.py:6135: error: "DataArray" has no attribute "isnull"

Edit:
one problem is certainly the method injection, as mypy cannot detect those types.

Start at the getattr definition in xarray/core/common.py and review the listed mypy errors in common.py, alignment.py, weighted.py, and dataset.py. Run the project's mypy checks after testing the proposed typing approach; done means missing methods such as reduce are reported while the listed valid method and attribute uses type-check cleanly.

Written by the indexing model from the issue text.

Don't type check getattr?

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