[python-package] fix mypy typing discrepancies in basic.py

[IdoKendo]

Contributes to #3867.
mypy currently raises the following errors in basic.py file:

python-package\lightgbm\basic.py:1127: error: Incompatible types in assignment (expression has type "_Pointer[c_int64]", variable has type "_Pointer[c_int32]")  [assignment]
python-package\lightgbm\basic.py:1132: error: Incompatible types in assignment (expression has type "_Pointer[c_double]", variable has type "_Pointer[c_float]")  [assignment]
python-package\lightgbm\basic.py:1444: error: Incompatible types in assignment (expression has type "c_void_p", variable has type "None")  [assignment]
python-package\lightgbm\basic.py:1448: error: Argument 1 to "byref" has incompatible type "None"; expected "_CData"  [arg-type]
python-package\lightgbm\basic.py:1499: error: Incompatible types in assignment (expression has type "c_void_p", variable has type "None")  [assignment]
python-package\lightgbm\basic.py:1510: error: Argument 1 to "byref" has incompatible type "None"; expected "_CData"  [arg-type]
python-package\lightgbm\basic.py:1595: error: Item "None" of "Optional[Dict[str, Any]]" has no attribute "get"  [union-attr]
python-package\lightgbm\basic.py:1689: error: Incompatible types in assignment (expression has type "c_void_p", variable has type "None")  [assignment]
python-package\lightgbm\basic.py:1694: error: Argument 1 to "byref" has incompatible type "None"; expected "_CData"  [arg-type]
python-package\lightgbm\basic.py:1827: error: Incompatible types in assignment (expression has type "c_void_p", variable has type "None")  [assignment]
python-package\lightgbm\basic.py:1842: error: Argument 1 to "byref" has incompatible type "None"; expected "_CData"  [arg-type]
python-package\lightgbm\basic.py:1857: error: Incompatible types in assignment (expression has type "Array[_Pointer[c_float]]", variable has type "Array[_Pointer[c_double]]")  [assignment]
python-package\lightgbm\basic.py:1883: error: Incompatible types in assignment (expression has type "c_void_p", variable has type "None")  [assignment]
python-package\lightgbm\basic.py:1887: error: Argument 1 to "c_int" has incompatible type "Optional[Any]"; expected "int"  [arg-type]
python-package\lightgbm\basic.py:1893: error: Argument 1 to "byref" has incompatible type "None"; expected "_CData"  [arg-type]
python-package\lightgbm\basic.py:1905: error: Incompatible types in assignment (expression has type "c_void_p", variable has type "None")  [assignment]
python-package\lightgbm\basic.py:1924: error: Argument 1 to "byref" has incompatible type "None"; expected "_CData"  [arg-type]
python-package\lightgbm\basic.py:1936: error: Incompatible types in assignment (expression has type "c_void_p", variable has type "None")  [assignment]
python-package\lightgbm\basic.py:1955: error: Argument 1 to "byref" has incompatible type "None"; expected "_CData"  [arg-type]
python-package\lightgbm\basic.py:2030: error: Incompatible types in assignment (expression has type "c_void_p", variable has type "None")  [assignment]
python-package\lightgbm\basic.py:2037: error: Argument 1 to "byref" has incompatible type "None"; expected "_CData"  [arg-type]
python-package\lightgbm\basic.py:2322: error: Value of type variable "SupportsRichComparisonT" of "sorted" cannot be "object"  [type-var]
python-package\lightgbm\basic.py:2565: error: Incompatible return value type (got "Union[List[Any], ndarray[Any, Any], Any, Any, None]", expected "Optional[ndarray[Any, 
Any]]")  [return-value]
python-package\lightgbm\basic.py:4256: error: Need type annotation for "values" (hint: "values: List[<type>] = ...")  [var-annotated]
python-package\lightgbm\basic.py:4326: error: No overload variant of "__setitem__" of "list" matches argument types "int", "ndarray[Any, dtype[floating[_64Bit]]]"  [call-overload]
python-package\lightgbm\basic.py:4326: note: Possible overload variants:
python-package\lightgbm\basic.py:4326: note:     def __setitem__(self, SupportsIndex, None, /) -> None
python-package\lightgbm\basic.py:4326: note:     def __setitem__(self, slice, Iterable[None], /) -> None
python-package\lightgbm\basic.py:4330: error: "None" has no attribute "ctypes"  [attr-defined]
python-package\lightgbm\basic.py:4336: error: Argument 1 to "len" has incompatible type "None"; expected "Sized"  [arg-type]
python-package\lightgbm\basic.py:4341: error: "None" has no attribute "size"  [attr-defined]
python-package\lightgbm\basic.py:4342: error: "None" has no attribute "reshape"  [attr-defined]
python-package\lightgbm\basic.py:4343: error: Incompatible return value type (got "Optional[Any]", expected "ndarray[Any, Any]")  [return-value]

These errors occur due to missing type annotations in the code and lack of None type validations.
This PR proposes fixing these errors by adding the missing annotations and None type checks.

Notes for Reviewers

This was tested by running mypy as documented in #3867.

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

[jameslamb]

Thanks!

For some of these proposed changes, I'll wait to review them until the other in-progress mypy and type hinting PRs that are currently open have been approved and merged.

For the changes about pointers...I'm nervous about taking a runtime dependency on things from ctypes which are prefixed with _, like ctypes._CData. If those things are removed in future releases of ctypes, import lightgbm will fail. That risk is not acceptable for the benefit of slightly more specific type hints.

What do you think about the approach I've proposed in #5729, to use string literals for such things to avoid runtime import errors in that situation?

[jameslamb]

@IdoKendo are you still interested in pursuing this PR?

If so, please see me comment above about using string literals for hints like ctypes._Pointer, and please update this branch to latest master.

If not (or if you'd prefer to work on something else), just let me know and I'll continue addressing these particular mypy errors.

[IdoKendo]

@IdoKendo are you still interested in pursuing this PR?

If so, please see me comment above about using string literals for hints like ctypes._Pointer, and please update this branch to latest master.

If not (or if you'd prefer to work on something else), just let me know and I'll continue addressing these particular mypy errors.

@jameslamb Unfortunately, not sure I have the capacity at this time, I'll close this PR so you can address it, and hopefully, soon I can get back to helping more.

[jameslamb]

ok no problem! Thanks so much for all your help so far, you've really helped re-invigorate this type annotation project that we'd been neglecting for a bit. Would love to have you come back and contribute any time.

[github-actions]

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