CLN refactor maybe-castable (#39257)

Author: MarcoGorelli

pandas/core/construction.py

@@ -588,9 +588,13 @@ def _try_cast(arr, dtype: Optional[DtypeObj], copy: bool, raise_cast_failure: bo
         Otherwise an object array is returned.
     """
     # perf shortcut as this is the most common case
-    if isinstance(arr, np.ndarray):
-        if maybe_castable(arr) and not copy and dtype is None:
-            return arr
+    if (
+        isinstance(arr, np.ndarray)
+        and maybe_castable(arr.dtype)
+        and not copy
+        and dtype is None
+    ):
+        return arr
 
     if isinstance(dtype, ExtensionDtype) and (dtype.kind != "M" or is_sparse(dtype)):
         # create an extension array from its dtype

pandas/core/dtypes/cast.py

@@ -1331,20 +1331,18 @@ def convert_dtypes(
     return inferred_dtype
 
 
-def maybe_castable(arr: np.ndarray) -> bool:
+def maybe_castable(dtype: np.dtype) -> bool:
     # return False to force a non-fastpath
 
-    assert isinstance(arr, np.ndarray)  # GH 37024
-
     # check datetime64[ns]/timedelta64[ns] are valid
     # otherwise try to coerce
-    kind = arr.dtype.kind
+    kind = dtype.kind
     if kind == "M":
-        return is_datetime64_ns_dtype(arr.dtype)
+        return is_datetime64_ns_dtype(dtype)
     elif kind == "m":
-        return is_timedelta64_ns_dtype(arr.dtype)
+        return is_timedelta64_ns_dtype(dtype)
 
-    return arr.dtype.name not in POSSIBLY_CAST_DTYPES
+    return dtype.name not in POSSIBLY_CAST_DTYPES
 
 
 def maybe_infer_to_datetimelike(