[python-package] fix mypy error about missing type hint in dask.py

[jameslamb]

Contributes to #3867.

This PR fixes one mypy error in dask.py.

python-package/lightgbm/dask.py:933: error: Need type annotation for "out"

Confirmed that this suppresses these warnings by running the following.

mypy \
    --exclude='python-package/compile/|python-package/build' \
    --ignore-missing-imports \
    python-package/

[jmoralez]

I think the first time out is populated in

LightGBM/python-package/lightgbm/dask.py

Line 945 in b94577d

out[i].append(part)

it's actually List[List[dask_Array]], where each entry has as many items as there are partitions. Then the contents get overwritten in

LightGBM/python-package/lightgbm/dask.py

Line 959 in b94577d

out[i] = dask_array_from_delayed(

where the partitions get concatenated and the elements become List[dask_Array].

I think variables changing types is a big no-no for mypy so maybe we could define two variables here?

[jameslamb]

it's actuallyList[List[dask_Array]]

ooooooo you're right! I missed that .append(). Fixed in 006a271.

where the partitions get concatenated and the elements become List[dask_Array].
I think variables changing types is a big no-no for mypy so maybe we could define two variables here?

You're generally right about mypy not liking it when code overwrites a variable with a value of a different type than it was initialized with. Fixed in 32f40de.

[jameslamb]

just re-ran the R-package jobs, let's see if the MiKTeX issue (#4005 (comment)) was fixed overnight

[jameslamb]

🎉 CI seems happy again!

e.g. from this Windows build

The downloaded binary packages are in
C:\Users\runneradmin\AppData\Local\Temp\RtmpwpHjaj\downloaded_packages
Downloading https://github.com/microsoft/LightGBM/releases/download/v2.0.12/miktexsetup-4.0-x64.zip
Setting up MiKTeX
Installing MiKTeX
Done installing MiKTeX

[github-actions]

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