[python-package] add type hints on some functions in `basic.py` (#5362)

* refactor: 🎨 add statuc typing to a subset of basic functions

* refactor: 🎨 add statuc typing to a subset of basic functions

* revert black format on python basic functions file

* style: 🎨

 updating typing for c_array function

* branch rebase

* fix number of changed lines

* Update python-package/lightgbm/basic.py
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: James Lamb <jaylamb20@gmail.com>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

python-package/lightgbm/basic.py

@@ -241,7 +241,7 @@ def _data_to_2d_numpy(data: Any, dtype: type = np.float32, name: str = 'list') -
                     "It should be list of lists, numpy 2-D array or pandas DataFrame")
 
 
-def cfloat32_array_to_numpy(cptr, length):
+def cfloat32_array_to_numpy(cptr: ctypes.POINTER, length: int) -> np.ndarray:
     """Convert a ctypes float pointer array to a numpy array."""
     if isinstance(cptr, ctypes.POINTER(ctypes.c_float)):
         return np.ctypeslib.as_array(cptr, shape=(length,)).copy()
@@ -249,7 +249,7 @@ def cfloat32_array_to_numpy(cptr, length):
         raise RuntimeError('Expected float pointer')
 
 
-def cfloat64_array_to_numpy(cptr, length):
+def cfloat64_array_to_numpy(cptr: ctypes.POINTER, length: int) -> np.ndarray:
     """Convert a ctypes double pointer array to a numpy array."""
     if isinstance(cptr, ctypes.POINTER(ctypes.c_double)):
         return np.ctypeslib.as_array(cptr, shape=(length,)).copy()
@@ -257,7 +257,7 @@ def cfloat64_array_to_numpy(cptr, length):
         raise RuntimeError('Expected double pointer')
 
 
-def cint32_array_to_numpy(cptr, length):
+def cint32_array_to_numpy(cptr: ctypes.POINTER, length: int) -> np.ndarray:
     """Convert a ctypes int pointer array to a numpy array."""
     if isinstance(cptr, ctypes.POINTER(ctypes.c_int32)):
         return np.ctypeslib.as_array(cptr, shape=(length,)).copy()
@@ -265,7 +265,7 @@ def cint32_array_to_numpy(cptr, length):
         raise RuntimeError('Expected int32 pointer')
 
 
-def cint64_array_to_numpy(cptr, length):
+def cint64_array_to_numpy(cptr: ctypes.POINTER, length: int) -> np.ndarray:
     """Convert a ctypes int pointer array to a numpy array."""
     if isinstance(cptr, ctypes.POINTER(ctypes.c_int64)):
         return np.ctypeslib.as_array(cptr, shape=(length,)).copy()
@@ -273,12 +273,12 @@ def cint64_array_to_numpy(cptr, length):
         raise RuntimeError('Expected int64 pointer')
 
 
-def c_str(string):
+def c_str(string: str) -> ctypes.c_char_p:
     """Convert a Python string to C string."""
     return ctypes.c_char_p(string.encode('utf-8'))
 
 
-def c_array(ctype, values):
+def c_array(ctype: type, values: List[Any]) -> ctypes.Array:
     """Convert a Python array to C array."""
     return (ctype * len(values))(*values)