Add type hints to core.py (#7707)

Co-authored-by: Chengyang Gu <bridgream@gmail.com>
Co-authored-by: jiamingy <jm.yuan@outlook.com>

python-package/xgboost/_typing.py

@@ -0,0 +1,60 @@
+"""Shared typing definition."""
+import ctypes
+import os
+from typing import Optional, List, Any, TypeVar, Union
+
+# os.PathLike/string/numpy.array/scipy.sparse/pd.DataFrame/dt.Frame/
+# cudf.DataFrame/cupy.array/dlpack
+DataType = Any
+
+# xgboost accepts some other possible types in practice due to historical reason, which is
+# lesser tested.  For now we encourage users to pass a simple list of string.
+FeatureNames = Optional[List[str]]
+
+ArrayLike = Any
+PathLike = Union[str, os.PathLike]
+CupyT = ArrayLike  # maybe need a stub for cupy arrays
+NumpyOrCupy = Any
+
+# ctypes
+# c_bst_ulong corresponds to bst_ulong defined in xgboost/c_api.h
+c_bst_ulong = ctypes.c_uint64  # pylint: disable=C0103
+
+CTypeT = Union[
+    ctypes.c_void_p,
+    ctypes.c_char_p,
+    ctypes.c_int,
+    ctypes.c_float,
+    ctypes.c_uint,
+    ctypes.c_size_t,
+]
+
+# supported numeric types
+CNumeric = Union[
+    ctypes.c_float,
+    ctypes.c_double,
+    ctypes.c_uint,
+    ctypes.c_uint64,
+    ctypes.c_int32,
+    ctypes.c_int64,
+]
+
+# c pointer types
+# real type should be, as defined in typeshed
+# but this has to be put in a .pyi file
+# c_str_ptr_t = ctypes.pointer[ctypes.c_char]
+CStrPtr = ctypes.pointer
+# c_str_pptr_t = ctypes.pointer[ctypes.c_char_p]
+CStrPptr = ctypes.pointer
+# c_float_ptr_t = ctypes.pointer[ctypes.c_float]
+CFloatPtr = ctypes.pointer
+
+# c_numeric_ptr_t = Union[
+#  ctypes.pointer[ctypes.c_float], ctypes.pointer[ctypes.c_double],
+#  ctypes.pointer[ctypes.c_uint], ctypes.pointer[ctypes.c_uint64],
+#  ctypes.pointer[ctypes.c_int32], ctypes.pointer[ctypes.c_int64]
+# ]
+CNumericPtr = ctypes.pointer
+
+# template parameter
+_T = TypeVar("_T")