Type errors in mypy GitHub Action

[lukeconibear]

What happened?

Running the GitHub Action for mypy locally returned errors:

$ python -m mypy --install-types --non-interactive 
xarray/core/merge.py:43: error: "tuple" is not subscriptable  [misc]
xarray/core/merge.py:44: error: "tuple" is not subscriptable  [misc]
xarray/core/merge.py:45: error: "tuple" is not subscriptable  [misc]
xarray/backends/api.py:62: error: "dict" is not subscriptable  [misc]
Found 4 errors in 2 files (checked 144 source files)

What did you expect to happen?

No type errors from mypy.

Minimal Complete Verifiable Example

No response

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS

commit: 3d3b236
python: 3.8.13 | packaged by conda-forge | (default, Mar 25 2022, 06:04:10)
[GCC 10.3.0]
python-bits: 64
OS: Linux
OS-release: 5.10.60.1-microsoft-standard-WSL2
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: 1.12.2
libnetcdf: 4.8.1

xarray: 2022.6.1.dev39+g3d3b236d
pandas: 1.4.3
numpy: 1.22.4
scipy: 1.9.0
netCDF4: 1.6.0
pydap: None
h5netcdf: 1.0.2
h5py: 3.7.0
Nio: None
zarr: 2.12.0
cftime: 1.6.1
nc_time_axis: 1.4.1
PseudoNetCDF: 3.2.2
rasterio: 1.3.2
cfgrib: 0.9.10.1
iris: 3.2.1
bottleneck: 1.3.5
dask: 2022.8.1
distributed: 2022.8.1
matplotlib: 3.5.3
cartopy: 0.20.3
seaborn: 0.11.2
numbagg: 0.2.1
fsspec: 2022.7.1
cupy: None
pint: 0.19.2
sparse: 0.13.0
flox: 0.5.9
numpy_groupies: 0.9.19
setuptools: 65.3.0
pip: 22.2.2
conda: None
pytest: 7.1.2
IPython: 8.4.0
sphinx: 5.1.1

[headtr1ck]

Which mypy version are you using?

[lukeconibear]

Which mypy version are you using?

0.971

[Illviljan]

Python 3.8 is used here. Is it something like this we're running into? https://stackoverflow.com/questions/62871524/is-pep-585-unusable-at-runtime-under-python-3-7-and-3-8

Maybe adding another mypy workflow for 3.8 would be a good idea?

[headtr1ck]

Strangely it seems to happen only as part of a Union definition.
Also we use tuple and dict all over the place and mypy does not complain anywhere else.

[headtr1ck]

I think we could just add a n additional mypy 3.8 workflow by using the arg --python-version 3.8