PERF: needs_i8_conversion expect dtype object (#52288)

Author: jbrockmendel

pandas/core/dtypes/common.py

@@ -1090,22 +1090,20 @@ def is_numeric_v_string_like(a: ArrayLike, b) -> bool:
     )
 
 
-def needs_i8_conversion(arr_or_dtype) -> bool:
+def needs_i8_conversion(dtype: DtypeObj | None) -> bool:
     """
-    Check whether the array or dtype should be converted to int64.
+    Check whether the dtype should be converted to int64.
 
-    An array-like or dtype "needs" such a conversion if the array-like
-    or dtype is of a datetime-like dtype
+    Dtype "needs" such a conversion if the dtype is of a datetime-like dtype
 
     Parameters
     ----------
-    arr_or_dtype : array-like or dtype
-        The array or dtype to check.
+    dtype : np.dtype, ExtensionDtype, or None
 
     Returns
     -------
     boolean
-        Whether or not the array or dtype should be converted to int64.
+        Whether or not the dtype should be converted to int64.
 
     Examples
     --------
@@ -1114,30 +1112,27 @@ def needs_i8_conversion(arr_or_dtype) -> bool:
     >>> needs_i8_conversion(np.int64)
     False
     >>> needs_i8_conversion(np.datetime64)
+    False
+    >>> needs_i8_conversion(np.dtype(np.datetime64))
     True
     >>> needs_i8_conversion(np.array(['a', 'b']))
     False
     >>> needs_i8_conversion(pd.Series([1, 2]))
     False
     >>> needs_i8_conversion(pd.Series([], dtype="timedelta64[ns]"))
-    True
+    False
     >>> needs_i8_conversion(pd.DatetimeIndex([1, 2, 3], tz="US/Eastern"))
+    False
+    >>> needs_i8_conversion(pd.DatetimeIndex([1, 2, 3], tz="US/Eastern").dtype)
     True
     """
-    if arr_or_dtype is None:
-        return False
-    if isinstance(arr_or_dtype, np.dtype):
-        return arr_or_dtype.kind in ["m", "M"]
-    elif isinstance(arr_or_dtype, ExtensionDtype):
-        return isinstance(arr_or_dtype, (PeriodDtype, DatetimeTZDtype))
-
-    try:
-        dtype = get_dtype(arr_or_dtype)
-    except (TypeError, ValueError):
+    if dtype is None:
         return False
     if isinstance(dtype, np.dtype):
-        return dtype.kind in ["m", "M"]
-    return isinstance(dtype, (PeriodDtype, DatetimeTZDtype))
+        return dtype.kind in "mM"
+    elif isinstance(dtype, ExtensionDtype):
+        return isinstance(dtype, (PeriodDtype, DatetimeTZDtype))
+    return False
 
 
 def is_numeric_dtype(arr_or_dtype) -> bool:

pandas/core/indexes/base.py

@@ -960,9 +960,7 @@ def view(self, cls=None):
             if isinstance(cls, str):
                 dtype = pandas_dtype(cls)
 
-            if isinstance(dtype, (np.dtype, ExtensionDtype)) and needs_i8_conversion(
-                dtype
-            ):
+            if needs_i8_conversion(dtype):
                 if dtype.kind == "m" and dtype != "m8[ns]":
                     # e.g. m8[s]
                     return self._data.view(cls)

pandas/core/reshape/merge.py

@@ -2015,7 +2015,7 @@ def _get_merge_keys(
                 f"with type {repr(lt.dtype)}"
             )
 
-            if needs_i8_conversion(lt):
+            if needs_i8_conversion(getattr(lt, "dtype", None)):
                 if not isinstance(self.tolerance, datetime.timedelta):
                     raise MergeError(msg)
                 if self.tolerance < Timedelta(0):
@@ -2101,7 +2101,7 @@ def injection(obj):
             raise ValueError(f"{side} keys must be sorted")
 
         # initial type conversion as needed
-        if needs_i8_conversion(left_values):
+        if needs_i8_conversion(getattr(left_values, "dtype", None)):
             if tolerance is not None:
                 tolerance = Timedelta(tolerance)
 

pandas/tests/dtypes/test_common.py

@@ -518,9 +518,12 @@ def test_needs_i8_conversion():
     assert not com.needs_i8_conversion(pd.Series([1, 2]))
     assert not com.needs_i8_conversion(np.array(["a", "b"]))
 
-    assert com.needs_i8_conversion(np.datetime64)
-    assert com.needs_i8_conversion(pd.Series([], dtype="timedelta64[ns]"))
-    assert com.needs_i8_conversion(pd.DatetimeIndex(["2000"], tz="US/Eastern"))
+    assert not com.needs_i8_conversion(np.datetime64)
+    assert com.needs_i8_conversion(np.dtype(np.datetime64))
+    assert not com.needs_i8_conversion(pd.Series([], dtype="timedelta64[ns]"))
+    assert com.needs_i8_conversion(pd.Series([], dtype="timedelta64[ns]").dtype)
+    assert not com.needs_i8_conversion(pd.DatetimeIndex(["2000"], tz="US/Eastern"))
+    assert com.needs_i8_conversion(pd.DatetimeIndex(["2000"], tz="US/Eastern").dtype)
 
 
 def test_is_numeric_dtype():